Patent Publication Number: US-7225048-B2

Title: Gap adjustment apparatus and gap adjustment method for adjusting gap between two objects

Description:
This is a Division of application Ser. No. 09/993,500 filed Nov. 27, 2001 now U.S. Pat. No. 6,852,988. The disclosure of the prior application(s) is hereby incorporated by reference herein in its entirety. 
   This application is based on Japanese Patent Applications 2000-219366 filed on Jul. 19, 2000, 2000-360502 filed on Nov. 28, 2000, and 2001-312624 filed on Oct. 10, 2001, the entire contents of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1) Field of the Invention 
   The present invention relates to a gap adjustment apparatus and a gap adjustment method, and more particularly, to a gap adjustment apparatus and a gap adjustment method suitable for adjusting a gap between a wafer and a mask (which are both used in X-ray lithography and electron beam exposure). 
   2) Description of the Related Art 
   In X-ray lithography and electron beam exposure, a mask is usually disposed in a position above a wafer (which will later be exposed) with a small gap formed between the wafer and the mask. In this manner, the surface of the wafer can be subjected to an exposure treatment by passing light through the mask. However, in order to improve the resolution and alignment precision, it is extremely important to precisely control the gap between the wafer and the mask. In particular, if such a gap is too large, a half-shadow problem will occur, resulting in reduced resolution as well as reduced alignment precision. 
   As a method for measuring the gap between the wafer and the mask, a method in which a high resolution camera is used to perform image processing, and another method in which a high resolution camera and electrostatic capacitance type sensors are used in combination are known. 
   However, since the electrostatic capacitance type sensors and the high-resolution camera are both quite expensive, if a gap measurement method employs both the electrostatic capacitance type sensors and the high-resolution camera, the overall apparatus for carrying out such a method will become expensive. In particular, an electrostatic capacitance type sensor capable of measuring a gap of 50 μm or smaller is extremely expensive. Moreover, it will be necessary to use expensive lens to photograph a gap of 20 μm or smaller. 
   In addition, a high-resolution camera usually requires a large space in order to be installed in an appropriate manner. For this reason, it will be difficult to manufacture a compact exposure apparatus. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an improved gap adjustment apparatus which is compact and which can be produced at low cost. 
   It is another object of the present invention to provide an improved method for measuring the aforementioned gap by using the improved gap adjustment apparatus. 
   According to one aspect of the present invention, there is provided a gap adjustment apparatus comprising: a first stage defining a first reference surface; a second stage defining a second reference surface, the second stage being opposite to the first stage in a manner such that the second reference surface becomes parallel to the first reference surface; a first chuck for fixing a first object having a main surface on the first stage, in a manner such that the main surface of the first object faces towards the second stage and becomes parallel to the first reference surface; a first displacement sensor attached on the first stage and adapted to measure a distance between the first displacement sensor and a certain plane disposed in front of the first displacement sensor and parallel to the second reference surface; a second chuck for fixing a second object having a main surface on the second stage, in a manner such that the main surface of the second object faces towards the first stage and becomes parallel to the second reference surface; a second displacement sensor attached on the second stage and capable of measuring a distance between the second displacement sensor and a certain plane disposed in front of the second displacement sensor and parallel to the first reference surface; an eddy current sensor attached on the first stage and having a sensor reference surface parallel to the first reference surface; an eddy current sensor target attached on the second stage and having a target reference surface parallel to the second reference surface; a moving mechanism for moving one of the first stage and the second stage in a direction parallel to the first reference surface and in another direction perpendicular to the first reference surface, with the movement of one stage being relative to the other stage; a controller which is adapted to drive the moving mechanism in a manner such that the target can be located in front of the first displacement sensor, to measure a distance extending from the first displacement sensor to the target reference surface of the target, to drive the moving mechanism in a manner such that the second main surface can be located in front of the first displacement sensor, to measure a distance extending from the first displacement sensor to the second main surface, to drive the moving mechanism in a manner such that the eddy current sensor can be located in front of the second displacement sensor, to measure a distance extending from the second displacement sensor to the sensor reference surface of the eddy current sensor, to drive the moving mechanism in a manner such that the first main surface can be located in front of the second displacement sensor, to measure a distance extending from the second displacement sensor to the first main surface, to drive the moving mechanism in a manner such that the target can be located in front of the eddy current sensor, and to measure an distance between the sensor reference surface and the target reference surface. 
   In this way, based on the difference between the distance extending from the first displacement sensor to the target reference surface and the distance extending from the first displacement sensor to the main surface of the second object, it is possible to determine a height difference between a height from the second reference surface to the target reference surface and a height from the second reference surface to the main surface of the second object. Similarly, it is possible to determine a height difference between a height from the first reference surface to the sensor reference surface and a height from the first reference surface to the main surface of the first object. Therefore, by using this difference information as well as the distance between the sensor reference surface and the target reference surface, it is possible to obtain the distance between the main surface of the first object and the main surface of the second object. 
   Furthermore, since the eddy current sensor is compact, even if the apparatus does not have sufficient internal space, such an eddy current sensor can still be disposed in a predetermined position. Moreover, since the eddy current sensor is relatively inexpensive, it is possible to manufacture the apparatus at low-cost. 
   According to another aspect of the present invention, there is provided a gap adjustment method comprising: a first step of fixing a first object on a first stage defining a first reference surface, in a manner such that a main surface of the first object becomes parallel to the first reference surface; a second step of fixing a second object on a second stage defining a second reference surface parallel to the first reference surface, in a manner such that a main surface of the second object becomes parallel to the second reference surface; a third step of calculating or adjusting a relationship between a sensor reference surface&#39;s height from the first reference surface and a height from the first reference surface to the main surface of the first object, a position of the sensor reference surface being fixed relative to a reference surface of an eddy current sensor attached on the first stage; a fourth step of calculating or adjusting a relationship between a target reference surface&#39;s height from the second reference surface and a height from the second reference surface to the main surface of the second object, the target reference surface being a target reference surface of an eddy current sensor target being fixed relative to the second stage; and a fifth step of measuring a distance extending from the sensor reference surface of the eddy current sensor to the target reference surface, and for adjusting a distance between the first stage and the second stage so as to enable a measurement result to be closer to a desired value. 
   In the above third step, it is possible to determine the relationship between a height from the first reference surface to the sensor reference surface and a height from the first reference surface to the main surface of the first object. In the above fourth step, it is possible to determine the relationship between a height from the second reference surface to the target reference surface and a height from the second reference surface to the main surface of the second object. Therefore, if the distance extending from the sensor reference surface to the target reference surface is known, it is possible to obtain the distance between the main surface of the first object and the main surface of the second object. 
   According to a further aspect of the present invention, there is provided a gap adjustment apparatus comprising: a mask chuck for holding a mask having a mask pattern formed thereon; a first leveling mechanism for holding the mask chuck and capable of moving the mask chuck in a first direction perpendicular to a surface on which the mask pattern of the mask fixed on the mask chuck has been formed; a mask stage for supporting the first leveling mechanism; a wafer chuck for holding the wafer in a manner such that an exposure surface of the wafer is caused to face towards the mask; a second leveling mechanism capable of moving the wafer chuck in the first direction; a wafer stage for holding the second leveling mechanism; a first distance sensor attached on the mask stage and capable of measuring a distance in the first direction, the distance extending to the exposure surface of the wafer held on the wafer chuck; and a second distance sensor attached on the wafer stage and capable of measuring a distance in the first direction, the distance extending to the surface of the mask fixed on the mask chuck, and also capable of measuring another distance in the first direction, the other distance extending to the first distance sensor. 
   When D A  represents a distance extending from the second distance sensor to the first distance sensor, D B  represents a distance extending from the second distance sensor to the surface of the mask, and D D  represents a distance extending from the first distance sensor to the exposure surface of the wafer, it is possible for the distance between the mask and the wafer to be represented by D D −(D A −D B ). The measured value is then compared with a desired value, while the first leveling mechanism or the second leveling mechanism is operated so that D D −(D A −D B ) becomes closer to the desired value, thereby effecting an adjustment of the distance between the mask and the wafer. 
   In this way, it becomes possible to measure the distance between the mask and the wafer at a relatively low cost and a high precision. 
   According to a still further aspect of the present invention, there is provided a gap adjustment method comprising: a step of disposing a first measurement object having a first surface and a second measurement object having a second surface so that the first surface and the second surface face each other, in a manner such that the first surface of the first measurement object and the second surface of the second measurement object become perpendicular to a first direction; a step of measuring a distance D A  in the first direction, the distance extending from a first distance sensor to a second distance sensor; a step of measuring a distance D B  in the first direction, the distance extending from the second distance sensor to the surface of the first measurement object; a step of measuring a distance D D  in the first direction, the distance extending from the first distance sensor to the second surface of the second measurement object; and a step of moving at least one of the first measurement object and the second measurement object in the first direction to make D D −(D A −D B ) closer to a desired value. 
   According to one more aspect of the present invention, there is provided a gap adjustment apparatus comprising: a first holding member for fixing and holding a first measurement object having a first surface; a first leveling mechanism capable of holding the first holding member and moving the first holding member in a first direction perpendicular to the first surface of the first measurement object fixed on the first holding member; a first stage for holding the first leveling mechanism; a second holding member for holding a second measurement object having a second surface, in a manner such that second surface faces towards the first surface; a second leveling mechanism capable of moving the second holding member in the first direction; a second stage for holding the second leveling mechanism; a first distance sensor attached on the first stage, capable of measuring a distance in the first direction extending to the second surface of the second measurement object fixed on the second holding member; and a second distance sensor attached on the second stage, for measuring a distance in the first direction extending to the first surface of the first measurement object fixed on the first holding member, and another distance in the first direction extending to the first distance sensor. 
   When D A  represents a distance extending from the second distance sensor to the first distance sensor, D B  represents a distance extending from the second distance sensor to the first surface of the first measurement object, and D D  represents a distance extending from the first distance sensor to the second surface of the second measurement object, it is possible for the distance between the first measurement object and the second measurement object to be represented by D D −(D A −D B ). The measured value is then compared with a desired value, while the first leveling mechanism or the second leveling mechanism is operated so that D D −(D A −D B ) becomes closer to the desired value, thereby effecting an adjustment of the distance between the first measurement object and the second measurement object. 
   According to another aspect of the present invention, there is provided a gap adjustment apparatus comprising: a first holder for holding a first object having a first measurement surface; a second holder for holding a second object having a second measurement surface, in a manner such that the second measurement surface faces the first measurement surface; a first displacement gauge for measuring a distance extending from the first displacement gauge to the first measurement surface; a target whose relative position with respect to the first displacement gauge is fixed, the target having a target measurement surface orientated in the same direction as the second measurement surface; a second displacement gauge for measuring distances extending from the second displacement gauge to the second measurement surface and to the target measurement surface; and a moving mechanism for moving at least one of the first holder and the second holder, in order to alter a gap between the first measurement surface and the second measurement surface. 
   With the use of the above gap adjustment apparatus formed according to the present invention, it is possible to measure in advance a distance extending from the first displacement gauge to the target measurement surface. In this way, by measuring a distance extending from the second displacement gauge to the target measurement surface, it is possible to calculate a distance between the first displacement gauge and the second displacement gauge. Therefore, it is possible to measure a gap between the first measurement surface and the second measurement surface, in accordance with a distance between the first displacement gauge and the first measurement surface, and a distance between the second displacement gauge and the second measurement surface, as well as a distance between the first displacement gauge and the second displacement gauge. 
   According to another aspect of the present invention, there is provided a gap adjustment method comprising the steps of: holding a first object having a first measurement surface and a second object having a second measurement surface, in a manner such that the second measurement surface is caused to face the first measurement surface; using a first displacement gauge to measure a distance D D  extending from the first displacement gauge to the first measurement surface; using a second displacement gauge to measure a distance D A  extending from the second displacement gauge to the second measurement surface; and using a distance D B  extending from the second displacement gauge to a target measurement surface whose relative position with respect to the first displacement gauge is fixed, a distance D C  extending from the first displacement gauge to the target measurement surface, as well as the distance D D  and the distance D A  to calculate D A +D D −(D B +D C ) and obtain a gap between the first measurement surface and the second measurement surface. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view showing an X-ray exposure apparatus formed according to a first embodiment of the present invention. 
       FIG. 2  is a flow chart showing a gap adjustment method according to a first embodiment of the present invention. 
       FIG. 3  is a schematic view showing a mask section and a wafer section of an X-ray exposure apparatus formed according to a second embodiment of the present invention. 
       FIG. 4  is a front view showing a mask chuck and several electrostatic capacitance type sensors, which are all attached on the mask stage of the X-ray exposure apparatus formed according to the second embodiment of the present invention. 
       FIG. 5  is an explanatory view schematically showing an electron beam proximity exposure equipment using a gap adjustment apparatus formed according to a third embodiment of the present invention. 
       FIG. 6A  is an explanatory view schematically showing the gap adjustment apparatus formed according to the third embodiment of the present invention. 
       FIG. 6B  is a front view of a mask. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a schematic view showing an X-ray exposure apparatus formed according to a first embodiment of the present invention. As shown in the drawing, a wafer stage  1  is provided to define a virtual wafer reference surface  3 , while a mask stage  2  is provided to define a virtual mask reference surface  4 . Here, the wafer stage  1  and the mask stage  2  are arranged to face each other, with the virtual wafer reference surface  3  and the virtual mask reference surface  4  being parallel to one another. 
   A wafer chuck  5  is mounted and installed on the wafer stage  1  by a moving mechanism  16 . The moving mechanism  16  is provided to move the wafer chuck  5  in the normal-line direction of the wafer reference surface  3 . Here, the wafer chuck  5  is adapted to adsorb the wafer  10  which will later be subjected to an exposure treatment. The main surface (to be later subjected to an exposure treatment) of the wafer  10  adsorbed by the wafer chuck  5 , is caused to face towards the mask stage  2  and is kept parallel to the wafer reference surface  3 . 
   A wafer-side laser displacement sensor  6  is attached on the wafer stage  1 . When a reflective surface for reflecting a laser beam is disposed in front of the wafer-side laser displacement sensor  6  in parallel to the mask reference surface  4 , the wafer-side laser displacement sensor  6  can operate to measure a distance between itself and the reflective surface. The measurement error of the laser displacement sensor  6  is 1 μm or less. 
   A mask chuck  7  is attached on the mask stage  2  for adsorbing and fixing the mask  11  (which is provided for use in an X-ray exposure treatment). A facing surface of the mask  11  adsorbed by the mask chuck  7  is caused to face towards the wafer stage  1 , while being parallel to the mask reference surface  4 . 
   A mask-side laser displacement sensor  8  is attached on the mask stage  2 . When a reflective surface for reflecting a laser beam is disposed in front of the mask-side laser displacement sensor  8  in parallel to the wafer reference surface  3 , the mask-side laser displacement sensor  8  can operate to measure a distance between itself and the reflective surface. The measurement error of the laser displacement sensor  8  is 1 μm or less. 
   An eddy current sensor  20  is attached on the wafer stage  1  by an actuator  21 . A target  25  for use with the eddy current sensor  20  is attached on the mask stage  2  by an actuator  26 . The eddy current sensor  20  has a sensor reference surface  22  in parallel with the wafer reference surface  3 . The target  25  has a target reference surface  27  in parallel with the mask reference surface  4 . When the target  25  is disposed in front of the eddy current sensor  20 , the eddy current sensor  20  can operate to measure the distance between the sensor reference surface  22  and the target reference surface  27 . The actuator  21  is adapted to adjust the height of the eddy current sensor  20  from the wafer reference surface  3 , while the actuator  26  is adapted to adjust the height of the target  25  suspended from the mask reference surface  4 . 
   Here, as the eddy current sensor  20 , EX-500 series sensors manufactured by Keyence Corp. can be used. The resolution capability of the EX-500 series sensors is 0.3 μm to 3.0 μm. Further, the target  25  may be formed by a metal plate having a thickness of 0.5 mm or more. 
   A moving mechanism  15  is adapted to move the wafer  1  in a two-dimensional plane which is parallel to the wafer reference surface  3 . Another moving mechanism  31  is adapted to move the mask stage  2  in a direction perpendicular to the mask reference surface  2 . The laser displacement sensors  6 , and  8 , the eddy current sensor  20 , the actuators  21 , and  26 , and the moving mechanisms  15 ,  16 , and  31  are all controlled by a controller  40 . 
   An X-ray light source  45  is provided to emit X-rays  46 . Such an X-ray light source  45  is a synchrotron, so that the X-rays  46  are synchrotron radiation light (SR light). Here, the X-rays  46  are emitted to pass through the mask  11  so as to irradiate the exposure surface (which will later be subjected to an exposure treatment) of the wafer  10 . 
   Next,  FIG. 2  (which is a flow chart) will be used to explain a method for carrying out an X-ray exposure treatment. As shown in the flow chart, at step S 1 , the wafer stage  1  is moved so that a predetermined area of the main surface of the wafer  10  is positioned in front of the mask-side laser displacement sensor  8 . Then, the mask-side laser displacement sensor  8  is operated to measure the distance between the mask-side laser displacement sensor  8  and the main surface of the wafer  10 . 
   Then, the step goes to step S 2  at which the wafer stage  1  is moved so that the eddy current sensor  20  is positioned in front of the mask-side laser displacement sensor  8 , thereby measuring the distance between the mask-side laser displacement sensor  8  and the sensor reference surface  22  of the eddy current sensor  20 . This measurement result is compared with the measurement result obtained at the above step S 1 , so as to adjust the height of the eddy current sensor  20  from the wafer reference surface  3 , in such a manner that the sensor reference surface  22  and the main surface of the wafer  10  will be positioned in exactly the same plane. In fact, such an adjustment may be carried out by driving the actuator  21 . 
   Subsequently, the step goes to step S 3  at which the wafer stage  1  is moved so that a predetermined area of the surface of the mask  11  is positioned in front of the wafer-side laser displacement sensor  6 . Then, the distance between the wafer-side laser displacement sensor  6  and the facing surface of the mask  11  is measured. 
   Afterwards, the step goes to step S 4  at which the wafer stage  1  is moved so that the target  25  is positioned in front of the wafer-side laser displacement sensor  6 , thereby measuring the distance between the wafer-side laser displacement sensor  6  and the target reference surface  27  of the target  25 . This measurement result is compared with the measurement result obtained at the above step S 3 , so as to adjust the height of the target  25  from the wafer reference surface  3 , in such a manner that the target reference surface  27  and the facing surface of the mask  11  will be positioned in exactly the same plane. In fact, such an adjustment may be carried out by driving the actuator  26 . 
   Then, the step goes to step S 5  at which the wafer stage  1  is moved so that the target  25  is positioned in front of the eddy current sensor  20 , thereby using the eddy current sensor  20  to measure the distance between the sensor reference surface  22  and the target reference surface  27 . Afterwards, the mask stage  2  is moved in a direction perpendicular to the mask reference surface  4 , so that the above distance can become close to a desired value. Here, the distance between the sensor reference surface  22  and the target reference surface  27  is equal to the distance between the main surface of the wafer  10  and the facing surface of the mask  11  (i.e., the distance between the wafer and the mask, or may be sometimes called a wafer-mask distance). In this way, the distance between the wafer and the mask can be made closer to the desired value. 
   Once the distance between the wafer and mask is in an allowable range, the step goes to step S 6  so as to perform an X-ray exposure treatment. 
   The above-described first embodiment shows that it is possible to adjust the distance between the main surface of the wafer  10  and the facing surface of the mask  11  with high precision, without having to use expensive electrostatic capacitance type sensors and an expensive high-resolution camera. Further, with the use of the eddy current sensor  20 , it is possible to constantly monitor the wafer-mask distance. In this way, when the wafer-mask distance deviates from a desired value, such a distance can be quickly returned to its original correct value. 
   Moreover, the laser displacement sensors and the eddy current sensor used in the first embodiment are all lightweight and compact. Therefore, even if the space surrounding the stages is not quite large enough, it is still possible to easily position these sensors without any difficulty. As a result, the total mechanical load on the stages can also be reduced. 
   Next, a modified embodiment will be described in relation to the above described first embodiment. In the first embodiment, at step S 2  shown in  FIG. 2 , the height of the sensor reference surface  22  was made coincident with the height of the main surface of the wafer  10 , while at step S 4  the height of the target reference surface  27  was made coincident with the height of the facing surface of the mask  11 . In the modified embodiment, at a step corresponding to step S 2  of the first embodiment, the controller operates to store a difference between the height of the sensor reference surface  22  and the height of the main surface of the wafer  10 . Further, at a step corresponding to step S 4  of the first embodiment, the controller operates to store a difference between the height of the target reference surface  27  and the height of the main surface of the wafer  10 . Thus, it is not necessary to adjust the height of the eddy current censor  20  and the height of the target  25 . 
   At a step corresponding to step S 5  of the first embodiment, a distance between the sensor reference surface  22  and the target reference surface  27  is measured. Thus, it is possible to calculate the actual wafer-mask distance, in accordance with a measured value of this distance between the sensor reference surface  22  and the target reference surface  27 , a difference between the height of the sensor reference surface  22  and the height of the main surface of the wafer  10 , as well as a difference between the height of the target reference surface  27  and the facing surface of the mask  11 . In this modified embodiment, it is not necessary to use the actuators  21  and  26  shown in  FIG. 1 . 
     FIG. 3  is a schematic view showing a distance adjustment apparatus formed according to a second embodiment of the present invention. As shown in the drawing, a mask stage  101  and a wafer stage  150  are arranged to be substantially parallel with each other, in a manner such that the mask holding surface and the wafer holding surface are in mutually facing positions. Further, an XYZ coordinate system is introduced in a manner such that its Z-axis is arranged in a direction perpendicular to the mutually facing surfaces of the mask stage  101  and the wafer stage  150 . 
   On the facing surface of the mask stage  101  there is provided a mask chuck  105  by a leveling mechanism  110 . A mask  103  having a predetermined mask pattern is vacuum adsorbed on the mask chuck  105 . The leveling mechanism  110  is adapted to adjust the height in the Z-axis direction, which height extends from the facing surface of the mask stage  101  to the mask  103  (i.e. the height in a direction perpendicular to the surface on which the mask pattern has been formed). Further, a plurality of electrostatic capacitance type sensors  107 A to  107 C are provided on the facing surface of the mask stage  101 . These electrostatic capacitance type sensors  107 A to  107 C, as shown in  FIG. 4 , are disposed in three positions corresponding to three corners of an imaginary equilateral triangle having a common center with the mask chuck  105 . Each of the electrostatic capacitance type sensors  107 A to  107 C is adapted to measure the distance from the corresponding electrostatic capacitance type sensor to an electrically conductive surface parallel to the X-Y plane. By actuating a two-axis moving mechanism  109 , the mask stage  101  can move in two axes parallel to the facing surface of the mask stage  101 , i.e., in the X direction as well as in the Y direction. 
   On the other hand, another two-dimensional moving mechanism  160  is installed on the facing surface of the wafer stage  150  by a leveling mechanism  158 . A wafer chuck  154  is attached to the two-dimensional moving mechanism  160 . The wafer chuck  154  is provided to vacuum adsorb the wafer  152 . With the wafer  152  being vacuum adsorbed by the wafer chuck  154 , the exposure surface of the wafer  152  will become parallel with the X-Y plane, so as to face the mask  103 . 
   The leveling mechanism  158  is adapted to adjust the height from the facing surface of the wafer stage  150  to the wafer  152 . The two-axis moving mechanism  160  is adapted to move the wafer  152  in two axes parallel to the exposure surface of the wafer, i.e. in the X direction as well as in the Y direction. Further, laser displacement sensors  156 A to  156 C are provided on the facing surface of the wafer stage  150 . These laser displacement sensors  156 A to  156 C are disposed in positions corresponding to the electrostatic capacitance type sensors  107 A to  107 C. Each of the laser displacement sensors  156 A to  156 C is adapted to measure a distance from the corresponding laser displacement sensor to a plane parallel to the X-Y plane, i.e. the distance in the Z direction. 
   The leveling mechanisms  110  and  158 , the two-axis moving mechanisms  109  and  160 , the electrostatic capacitance type sensors  107 A to  107 C, and the laser displacement sensors  156 A to  156 C are all controlled by a controller  170 . X-rays  120  are for example synchrotron radiation light which irradiates the mask  103  on the mask stage  101 , thereby transferring a pattern formed on the mask  103  to the exposure surface of the wafer  152 . 
   Next, a detailed description will be given to explain a method for adjusting a gap between the mask  103  and the wafer  152  in the X-ray exposure apparatus shown in  FIG. 3 . 
   Here, it is supposed that the calibration of the three laser displacement sensors  156 A to  156 C, as well as the calibration of the three electrostatic capacitance type sensors  107 A to  107 C have all been finished. Namely, when the distance from the facing surface of the wafer stage  150  to a plane positioned at a certain height from the facing surface has been measured, the measured values produced by the laser displacement sensors  156 A to  156 C should all be coincident with one another. Further, when the distance from the facing surface of the mask stage  101  to a plane positioned at a certain height from the facing surface has been measured, the measured values produced by the electrostatic capacitance type sensors  107 A to  107 C should all be coincident with one another. 
   The mask  103  is fixed on the mask chuck  105 , while the wafer  152  is fixed on the wafer chuck  154 . At three positions not arranged along a straight line on the exposure surface of the wafer  152 , the distance is between the exposure surface and the three electrostatic capacitance type sensors  107 A to  107 C is measured. Then, the leveling mechanism  158  is driven so as to make all the measured results equal to one another. Further, at three positions not arranged along a straight line on the surface of the mask  103 , any one of the laser displacement sensors  156 A to  156 C is used to measure the distance between the corresponding laser displacement sensor and the mask surface. Then, the leveling mechanism  110  is driven so as to make all three measured results equal to one another. Subsequently, the two-axis moving mechanism  109  is operated in a manner such that the measurement points on the surface of the mask  103  can be located in front of the laser displacement sensors, thereby measuring the distances between the laser displacement sensors and the measurement points. In this way, a plane on which the mask pattern of the mask  103  has been formed can be made parallel to the exposure surface of the wafer  152 . Moreover, since there are provided three electrostatic capacitance type sensors  107 A to  107 C and three laser displacement sensors  156 A to  156 C, when the leveling step is being carried out, it is possible to reduce the distances moved by both the mask  103  and the wafer  152  in both the X direction and the Y direction. 
   During the leveling step, a distance D B  between the laser displacement sensor  156 A and the surface of the mask  103 , i.e., extending in the Z-axis direction, is measured. 
   Afterwards, the two-axis moving mechanism  109  is operated to have the electrostatic capacitance type sensor  107 A located in front of the laser displacement sensor  156 A. Then, the laser displacement sensor  156 A is used to measure a distance D A  extending from the laser displacement sensor  156 A to the electrostatic capacitance type sensor  107 A. However, since the distance D A  does not change even if the mask  103  and the wafer  152  are replaced by new ones, it is not necessary to perform measurement every time the mask  103  and the wafer  152  are replaced. The difference between the distance D A  and the distance D B  corresponds to a distance D C  between the surface of the electrostatic capacitance type sensor  107 A and the mask  103 . 
   Then, the electrostatic capacitance type sensor  107 A is used to measure a distance D D  between the surface of the electrostatic capacitance type sensor  107 A and the exposure surface of the wafer  152 , i.e., extending in the Z-axis direction. A gap G between the surface of the mask  103  and the exposure surface of the wafer  152  can be calculated in accordance with the following equation:
 
 G=D   D   −D   C   =D   D −( D   A   −D   B )
 
   Here, the difference between the desired value and the gap G obtained according to the above equation may be calculated. By driving the leveling mechanism  158 , the wafer  152  can be moved only by a distance corresponding to the difference. In the above step, the distance between the mask  103  and the wafer  152  can be adjusted to make the distance value close to the desired value. 
   Preferably, it is desired that the gap G does not change even if the two-axis moving mechanism  160  is operated. However, in fact, the distance G will still change a little within the allowable precision range of the two-axis moving mechanism  160 . On the other hand, even if the two-axis moving mechanism  160  is actuated, the distance D A  and the distance D B  will not change. For this reason, after the two-axis moving mechanism  160  has been actuated and an exposure area of the exposure surface of the wafer  152  has been moved, it is possible to detect a change of the gap G and to compensate for a deviation of the gap G by re-measuring only the distance D D . 
   With the use of the above X-ray exposure apparatus formed according to the second embodiment of the present invention, it is possible to correctly measure the distance between the mask and the wafer without having to use a high-resolution camera. Further, since it is not necessary to employ an expensive and large high-resolution camera, it is possible to manufacture an improved exposure apparatus which is compact and low-cost. 
   The measurement error of each laser displacement sensor is about ±0.1 μm, while the measurement error of each electrostatic capacitance type sensor is ±50 nm. Accordingly, each of the distances D A  and D B  will contain at most an error of ±0.1 μm, while the distance D D  will contain at most an error of ±0.01 μm. In this way, it is possible to obtain the gap G with a precision of about ±0.2 μm. 
   Although it has been described in the above second embodiment that there are provided three electrostatic capacitance type sensors  107 A to  107 C and three laser displacement sensors  156 A to  156 C, it is possible to provide only one electrostatic capacitance type sensor and only one laser displacement sensor. By operating the two-axis moving mechanism  160 , it is possible to measure the distances between the electrostatic capacitance type sensors and the wafer  152  at three positions on the exposure surface of the wafer  152 . Similarly, by operating the two-axis moving mechanism  109 , it is possible to measure the distances between the laser displacement sensors and the mask  103  at three positions on the surface of the mask  103 . 
   Moreover, although it has been described in the above second embodiment that the laser displacement sensors are disposed on the wafer stage  150  and the electrostatic capacitance type sensors are disposed on the mask stage  101 , it is also possible to dispose other types of distance sensors. For example, if an electrically conductive film is formed on an area (not containing the mask pattern) on the surface of the mask  103 , it is possible that electrostatic capacitance type sensors can be used to replace the laser displacement sensors. Since the electrostatic capacitance type sensors have a lower price, a higher precision, and a smaller size than the laser displacement sensors, it is possible to manufacture an exposure apparatus which is compact and which can be produced at low cost. 
   In the second embodiment, the mask chuck  105  is mounted on the mask stage  101  via the leveling mechanism  110 . However, the mask chuck  105  may be fixed to the mask stage  101  without the leveling mechanism  110 . In such case, the mask stage itself is moved in the Z-axis direction by another leveling mechanism. When the mask chuck  105  is fixed to the mask stage  101 , the distance Dc does not change as long as the mask  103  is not changed. 
     FIG. 5  is an explanatory view showing an electron beam approaching exposure equipment  210  involving the use of a gap adjustment apparatus formed according to the third embodiment of the present invention. 
   As shown in the drawing, the electron beam approaching exposure equipment  210  comprises an electron gun  212 , a beam scanning section  220  and a gap adjustment apparatus  230 . The electron gun  212  includes an electron beam source  214 , an electrostatic lens  216  and an aperture  218 . The electron beam source  214  is adapted to emit an electron beam. In fact, the electrostatic lens  216  is provided to convert the electron beam emitted from the electron beam source  214  into a collimated beam  215 . The collimated beam  215  is then shaped by the aperture  218  and allowed to enter the beam scanning section  220 . 
   The beam scanning section  220  comprises main deflectors  222  and  224  as well as auxiliary deflectors  226  and  228 . In this way, the beam scanning section  220  can perform a desired beam scanning. 
   The gap adjustment apparatus  230  is so formed that it can keep a mask  231  in a position through which the scanning electron beam is passing. A wafer  240  is arranged to face the mask  231 , with a small gap (a proximity gap) formed therebetween. A resist film  242  to be exposed by the electron beam is formed on the wafer  240 &#39;s one surface facing the mask  231 . In this manner, the electron beam passing through the mask  231  may be incident on the resist film  242 , thereby enabling the pattern of the mask  231  to be transferred to the resist film  242 . 
     FIG. 6A  is another explanatory view showing in more detail the gap adjustment apparatus  230  formed according to the present embodiment of the present invention. As shown in the drawing, a wafer stage  251  is held by an XY driving mechanism  250 , in a manner such that the wafer stage is movable in a two-dimensional plane which is parallel to the XY plane. A wafer chuck  253  is attached to the wafer stage  251  through three Z-direction driving mechanisms  252 . In fact, the three Z-direction driving mechanisms  252  are provided to move the wafer chuck  253  in the Z-direction perpendicular to the XY-plane. In particular, the three Z-direction driving mechanisms  252  are so formed that their driving amounts can be different from one another. As a result, an inclining angle of the wafer chuck  253  with respect to the XY-plane may be properly adjusted. In use, the wafer chuck  253  can hold a wafer  240  by means of an electrostatic chuck mechanism. Actually, the XY driving mechanism  250  and the Z-direction driving mechanism  252  are provided to operate in accordance with an instruction issued by a controller  270 . 
   A mask chuck  260  is provided to hold the mask  231  by means of an electrostatic mechanism. The mask  231  held by the mask chuck  260  is arranged to face the wafer  240  held by the wafer chuck  253 , in a manner such that an extremely small gap is formed between the wafer  240  and the mask  231 . Here, two mutually facing surfaces of the wafer  240  and the mask  231  are referred to as a measurement surface  240   a  and a measurement surface  231   a . Specifically, at least one portion of each of the measurement surfaces  231   a  and  240   a  is formed by an electrically conductive material, with each electrically conductive portion connected to an earth potential. For example, the measurement surface  240   a  of the wafer  240  is made of silicon. Silicon can be regarded as an electrically conductive material. Further, a resist film to be subjected to the electron beam exposure is formed on the measurement surface  240   a  of the wafer  240 . 
   The mask chuck  260  is connected to a mask stage  261  by means of three Z-direction driving mechanisms  262  which are adapted to move the mask chuck  260  in the Z-direction. In particular, the three Z-direction driving mechanisms  262  are so formed that their driving amounts are different from one another. As a result, an inclining angle of the mask chuck  260  with respect to the XY-plane can be properly adjusted. Actually, these Z-direction driving mechanisms  262  will operate in accordance with an instruction issued by a controller  270 . 
   An electrostatic capacity type displacement gauge (electrostatic capacitance type sensor)  255  for use with the mask  231  is attached on the wafer stage  251 . Here, the electrostatic capacity type displacement gauge  255  is arranged to face the mask  231 . On the other hand, another electrostatic capacity type displacement gauge  265  for use with the wafer as well as a target  266  is attached on the mask stage  261 . Specifically, the target  266  has a target measurement surface  266   a  facing in the same direction as the measurement surface  231   a  of the mask  231 . Further, the target measurement surface  266   a  is formed by an electrically conductive material and connected to an earth potential. A Z-direction distance D C  between the electrostatic capacity type displacement gauge  265  and the target measurement surface  266   a  is measured correctly in advance with the use of a laser displacement gauge. In particular, the target  266  is fixed relatively with respect to the electrostatic capacity type displacement gauge  265 . As a result, the distance D C  is not variable once the setting of the apparatus is completed. 
     FIG. 6B  is a front view of the mask  231  of NIST format. As shown in the drawing, the mask  231  includes an annular frame  232  formed by aluminum and a support plate  233  formed by silicon. A square membrane  234  having a pattern to be transferred is held on the central portion of the support plate  233 . In fact, the surface of the support plate  233  serves to define the measurement surface  231   a  of the mask  231 . Here, each of the four edges of the square membrane  234  has a length of 40 to 50 mm, while the support plate  233  has a diameter of 125 mm. Specifically, with regard to the support plate  233 , an annular area  233   a  having a width of about 10 mm surrounding the membrane  234  is an area which is so formed that its displacement amount can be detected by means of the electrostatic capacity type displacement gauge. 
   In this way, by driving the XY driving mechanism  250  and by disposing the electrostatic capacity type displacement gauge  255  on the front of any portion of the annular area  233   a , it is possible to measure a displacement amount of the mask  231  at the time when the reference surface of the electrostatic capacity type displacement gauge  255  is used as an original point of the displacement amount. Here, a distance between the electrostatic capacity type displacement gauge  255  and its reference surface may be determined in advance. Therefore, the measuring of the displacement amount of the measurement surface  231   a  is equivalent to the measuring of a distance D A  between the electrostatic capacity type displacement gauge  255  and the measurement surface  231   a . In the present specification, a process using an electrostatic capacity type displacement gauge to measure a displacement amount extending from the reference surface to a measurement surface is referred to as “measuring a distance extending from the electrostatic capacity type displacement gauge to the measurement surface”. 
   Here, by driving the XY driving mechanism  250  and by disposing the electrostatic capacity type displacement gauge  255  on the front surface of the target  266 , it is possible to measure a distance D B  between the electrostatic capacity type displacement gauge  255  and the target measurement surface  266   a . In this way, by driving the XY driving mechanism  250  and by disposing the wafer chuck  253  in a manner such that the electrostatic capacity type displacement gauge  265  may be positioned on the front surface of the wafer  240 , it is possible to measure a distance D D  between the electrostatic capacity type displacement gauge  265  and the measurement surface  240   a  of the wafer  240 . The measurement results of the electrostatic capacity type displacement gauges  255  and  265  are fed into the controller  270 . 
   In general, a measurable distance of the electrostatic capacity type displacement gauge is about 0.5 mm or more. Further, with regard to the mask  231 , when an area  233   a  serving as the target of the electrostatic capacity displacement gauge  255  has a width of about 10 mm, a measurable distance of the electrostatic capacity type displacement gauge  255  is for example about 1.5 mm or less. For this reason, as an example, the distance D A  and the distance D D  are set to be about 1 mm, while the distance D B  is set to be about 1.3 mm. 
   Next, description will be given to explain a method for adjusting a gap between the mask  231  and the wafer  240 , using a gap adjustment apparatus shown in  6 A. 
   At first, distances are measured which are extending between the electrostatic capacity type displacement gauge  255  and at least three points on the measurement surface  231   a  of the mask  231 . Then, in accordance with the measurement results, the Z-direction driving mechanism  262  is operated in a manner such that the measurement surface  231   a  of the mask  231  becomes parallel with the XY plane. In this way, after the measurement surface  231   a  has been made parallel with the XY plane, the distance D A  is thus measured. Further, the distance D B  is also measured. 
   Subsequently, distances are measured which are extending between the electrostatic capacity type displacement gauge  265  and at least three points on the measurement surface  240   a  of the wafer  240 . Then, in accordance with the measurement results, the Z-direction driving mechanism  252  is operated in a manner such that the measurement surface  240   a  of the wafer  240  becomes parallel to the XY plane. In this way, after the measurement surface  240   a  has been made parallel with the XY plane, the distance D D  can thus be measured. 
   A gap between the mask  231  and the wafer  240  may be represented by the following equation.
 
 G=D   A   +D   D −( D   B   +D   C )  (1)
 
   At this time, the Z-direction driving mechanism  252  or the Z-direction driving mechanism  262  is operated in a manner such that the gap G will arrive at a desired value. Therefore, it is possible to measure the gap G. 
   Preferably, the distance D B  is measured only once and there is no variation thereafter. For this reason, it is not necessary to again perform the measurement when the mask  231  and the wafer  240  are changed. However, since the distance D B  will change with the passing of time, it is preferred to periodically measure the distance D B  and renew it into a new value. 
   In the above embodiment, only the electrostatic capacity type displacement gauges are used as the displacement gauges. Since each electrostatic capacity type displacement gauge does not generate a magnetic field during a period when distance measurement is not being performed, it does not produce any influence in the proceeding direction of the electron beam. Further, since the electrostatic capacity type displacement gauge is simpler in its structure than a laser displacement gauge and does not contain a print board which is often responsible for degassing, the gap adjustment apparatus of the present embodiment is suitable for use in a vacuum atmosphere, such as in an electron beam approaching exposure apparatus. Moreover, since the gap adjustment apparatus of the present embodiment is not required to incorporate high price and high resolution camera, it is allowed to manufacture the entire apparatus at a reduced cost. 
   Here, taking into account the characteristics of the electrostatic capacity type displacement gauge, a distance D D  between the electrostatic capacity type displacement gauge  265  and the wafer  240  is preferred to be set at about 1 mm. For the same reason, a distance D A  between the electrostatic capacity type displacement gauge  255  and the mask  231  is also preferred to be set at about 1 mm. A gap G between the wafer  240  and the mask  231  is usually set at about 50 μm. For this reason, a distance D B +D C  between the two electrostatic capacity type displacement gauges  255  and  265  will become about 2 mm, and will extend beyond the distance measurable range of the electrostatic capacity type displacement gauges. As a result, it is impossible to directly measure the right side D B +D C  of equation (1). Even if the data indicating the distance D B +D C  was obtained, the data would include a large measurement error because the area of the front surface of the electrostatic capacity type displacement gauge is not large enough to be used as an object to be measured by the electrostatic capacity type displacement gauge. 
   In the case of the above embodiment, by disposing the target measurement surface  266   a  of the target  266  within the distance measurable range of the electrostatic capacity type displacement gauge  255 , it is possible to measure a distance D B +D C  between the two electrostatic capacity type displacement gauges  255  and  265 . 
   Further, although it has been described in the above embodiment that the electrostatic capacity type displacement gauges are used as displacement gauges, it is also possible to use optical fiber type displacement gauges. An optical fiber type displacement gauge is formed in a manner such that a light is emitted from end face of optical fiber and such a light is incident on an object whose distance is to be measured. Then, the light is reflected from the object and the reflected light is received, thereby allowing the desired distance measurement in accordance with an amount of the received light. In the case where an optical fiber type displacement gauge is utilized, it is not absolutely necessary to dispose the target  266 . 
   Thus, in the case where the target  266  is not disposed, it is also possible to directly measure the distance D B +D C  between the two displacement gauges  255  and  265  shown in  FIG. 6A . If the directly measured distance between the two displacement gauges  255  and  265  is defined as D E , a gap G can be represented in the following equation.
 
 G=D   A   +D   D   −D   E    (2)
 
   Although the present invention has been described in accordance with the above embodiments, this invention should not be limited to those specific embodiments. An ordinary person skilled in the art can easily perform any modification, improvement and combination based on the above embodiments.