Abstract:
A sample micromotion mechanism adapted to minimize an influence of a disturbance and adjust a sample drift rapidly and with high accuracy, and designed so as to be a compact, easy-to-place sample micromotion mechanism of a side-entry type that suppresses the occurrence of the sample drift and generates/displays high-resolution monitoring images and precisely drawn patterns. A charged particle device employing the sample micromotion mechanism operates followed by deformation which causes a strain. A strain measuring unit measures such strain. The sample micromotion mechanism imparts micromotion so as to reduce the strain in accordance with the measured strain value, thereby reducing deformation of the sample micromotion mechanism.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese patent application JP 2014-045340 filed on Mar. 7, 2014, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates generally to sample micromotion mechanisms, methods of using the same, and charged particle devices. More particularly, the invention relates to sample micromotion mechanisms each equipped in a charged particle device that uses charged particle beams, for example, to monitor a surface shape and internal state of a substance or to draw patterns on the substance surface. The invention is further directed to methods of using such a sample micromotion mechanism. 
     Description of the Related Arts 
     The charged particle devices that use charged particle beams to monitor a surface shape and internal state of a substance or to draw patterns on the substance surface can provide high-resolution monitoring images and precisely drawn patterns. Sample micromotion mechanisms employed in such charged particle devices are required to have advantages such as ease of sample-loading operations, wide irradiation angles when used to irradiate a sample with charged particle beams, and wide angles for detecting the charged particle beams reflected/scattered from the surface of the sample. For these reasons, sample micromotion mechanisms called the side-entry type are mainly used in recent years. 
     In the sample micromotion mechanisms of the side-entry type, after a sample has been positioned, the positioned sample occasionally continues to move slightly. This event is called the sample drift. If the sample drift occurs, a monitoring image will blur, which may cause difficulty in obtaining a fine image, may deform a drawn pattern, and thus may reduce accuracy. 
     The following techniques are disclosed to suppress the occurrence of the above-discussed sample drift. 
     JP-A-2009-81080 (Patent Document 1) discloses a method of providing a plurality of places at which a spherical fulcrum and a spherical seat corresponding thereto come into mutual contact, and thereby stabilizing the contact between both and preventing deformation of a sample micromotion mechanism from changing with an elapse of time. 
     JP-A-2004-259448 (Patent Document 2) discloses a method of reducing deformation of a sample micromotion mechanism due to movement thereof, by using a repulsive force of an O-ring generated by its own deformation. 
     JP-A-2006-114251 (Patent Document 3) discloses a technique applied to a transmission electron microscope that is one type of charged article device, the technique including a method of moving and/or adjusting a sample micromotion mechanism so as to detect, directly with a sensor, a drift of a pattern of a charged particle beam transmitted through a sample, and correct the drift. 
     SUMMARY OF THE INVENTION 
     The techniques discussed above, however, has the following problems. 
     The method proposed in Patent Document 1 to provide the plurality of places at which the spherical fulcrum and the spherical seat come into mutual contact leaves the problem of sample drift unsolved, if a disturbance occurs that changes the contact state. 
     The method proposed in Patent Document 2 to reduce the deformation of the sample micromotion mechanism by using the repulsive force of the O-ring generated by its own deformation poses a problem in that if the distance through which the sample micromotion mechanism has moved is too short, probable lack of the repulsion may result in failure to sufficiently reduce the deformation or create a need to spend a great amount of time before the deformation can be reduced to a certain level. 
     The method proposed in Patent Document 3 to move and/or adjust the sample micromotion mechanism so as to detect the drift directly with the sensor and correct the drift requires a light-receiving time to be extended for pattern detection with the sensor and thus presents a problem in that it takes a long time to complete the correction of the drift by the movement and/or adjustment of the sample micromotion mechanism. 
     Additionally, if the sample drift occurs after the operations on the sample micromotion mechanism, a person who is observing the state of the substance uses the sample micromotion mechanism to operate its sample micromotion machine through a certain distance in a direction reverse to a previous direction in which the machine operations have been performed up until that time. Such operations are called “counter operations”, which tend to be excessive and/or insufficient operations in a manual operation mode. 
     Furthermore, since the sample micromotion mechanisms of the side-entry type are each placed so as to cross a magnetic field that an objective lens generates, the sample micromotion mechanism itself is reduced in size and makes it difficult to place an element that measures the amount of deformation of the sample micromotion mechanism. 
     Accordingly, an object of the present invention is to provide a sample micromotion mechanism of the side-entry type that is adapted to minimize an influence of a disturbance and adjust the sample drift rapidly with high accuracy, and that is designed to be compact and easy-to-place. Thus, the occurrence of the sample drift can be suppressed and high-resolution monitoring images and precisely drawn patterns are obtained. The invention is also intended to provide a method of using such a sample micromotion mechanism, and a charged particle device employing the sample micromotion mechanism. 
     Means for Solving the Problems 
     Some of major features of the charged particle device in an aspect of the present invention for solving the above problems are outlined below. 
     The charged particle device includes: a charged particle beam source that generates a charged particle beam; a charged particle optical system that controls a path for the charged particle beam to propagate; a sample holder that supports a sample irradiated with the charged particle beam; a sample micromotion mechanism that imparts micromotion to the sample holder and positions the sample; and a sensor unit that detects the charged particle beam transmitted through the sample; wherein the sample micromotion mechanism includes a strain measuring unit that measures strain in a member constituting the sample micromotion mechanism, and after positioning the sample, the sample micromotion mechanism imparts the micromotion to the sample holder so as to counteract the strain in accordance with the strain measured by the strain measuring unit. 
     In addition, some of major features of the sample micromotion mechanism in an aspect of the present invention are outlined below. 
     The sample micromotion mechanism includes: a sample holder that supports a sample observed through a charged particle device; a sliding cylinder that accommodates the sample holder; a micromotion mechanism that imparts micromotion to a distal end portion of the sample holder supporting the sample; and a strain measuring unit that measures the amount of strain occurring between the sliding cylinder and the micromotion mechanism. 
     Furthermore, some of major features of a usage method for the sample micromotion mechanism in an aspect of the present invention are outlined below. 
     In the sample micromotion mechanism including a sample holder that supports a sample observed through a charged particle device, a sliding cylinder that accommodates the sample holder, a micromotion mechanism that imparts a micromotion to a distal end portion of the sample holder supporting the sample, and a strain measuring unit that measures the amount of strain occurring between the sliding cylinder and the micromotion mechanism, the micromotion mechanism employs a sample micromotion mechanism that includes a rotary mechanism to impart a rotation to the distal end portion of the sample holder, a horizontal micromotion mechanism to impart to the distal end portion a micromotion directed along a lateral surface of the sliding cylinder, and a microswinging motion mechanism to swing the distal end vertically and horizontally about a central axis of the sample holder; wherein the method of using the sample micromotion mechanism includes (1) operating the sample micromotion mechanism and positioning the sample holder, (2) stopping the operation of the sample micromotion mechanism after the positioning, (3) blocking a charged particle beam generated by the charged particle device, (4) activating the rotary mechanism to minimize the strain caused by deformation of the sliding cylinder, (5) activating the microswinging motion mechanism to swing the distal end vertically and horizontally for minimizing the strain caused by the deformation of the sliding cylinder, (6) activating the horizontal micromotion mechanism to impart micromotion in a horizontal direction for minimizing the strain caused by the deformation of the sliding cylinder, (7) minimizing the strain caused by the deformation of the sliding cylinder, and (8) releasing a blocked state of the charged particle beam. 
     In accordance with the present invention, the sample micromotion mechanism can be provided that is adapted to minimize the influence of a disturbance and adjust the sample drift rapidly and with high accuracy, and designed so as to be the compact, easy-to-place sample micromotion mechanism of the side-entry type that suppresses the occurrence of the sample drift and generates/displays high-resolution monitoring images and precisely drawn patterns. In addition, the method of using the sample micromotion mechanism, and the charged particle device employing the sample micromotion mechanism can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a transmission electron microscope having a sample micromotion mechanism according to the present invention. 
         FIG. 2  is a sectional view of a sample micromotion mechanism of a charged particle device shown and described as a first embodiment of the present invention. 
         FIG. 3  is another sectional view of the sample micromotion mechanism of the charged particle device shown and described as the first embodiment of the present invention. 
         FIG. 4  is yet another sectional view of the sample micromotion mechanism of the charged particle device shown and described as the first embodiment of the present invention. 
         FIG. 5  is a further sectional view of the sample micromotion mechanism of the charged particle device shown and described as the first embodiment of the present invention. 
         FIG. 6  is a further sectional view of the sample micromotion mechanism of the charged particle device shown and described as the first embodiment of the present invention. 
         FIG. 7  is an outline diagram of strain detection in a sample holder shown in the first embodiment of the present invention. 
         FIG. 8  is a diagram showing an example of a semiconductor resistance strain sensor for detecting strain. 
         FIG. 9  is a diagram showing another example of a semiconductor resistance strain sensor for detecting strain. 
         FIG. 10  is a diagram showing yet another example of a semiconductor resistance strain sensor for detecting strain. 
         FIG. 11  is a flowchart that shows operation of a charged particle device according to the present invention. 
         FIG. 12  is a cross-sectional view of a sample micromotion mechanism of a charged particle device shown and described as a second embodiment of the present invention. 
         FIG. 13  is a diagram representing one example of a relationship between changes in a controlled variable of a motor shown and described in the second embodiment of the present invention, and changes in strain data detected for the controlled variable. 
         FIG. 14  is a diagram representing another example of a relationship between changes in a controlled variable of a motor shown and described in the second embodiment of the present invention, and changes in strain data detected for the controlled variable. 
         FIG. 15  is a diagram representing yet another example of a relationship between changes in a controlled variable of a motor shown and described in the second embodiment of the present invention, and changes in strain data detected for the controlled variable. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following describes embodiments of the present invention. More specifically, the description is given of a configuration, functions, and advantageous effects of each of the embodiments as achieved when a sample micromotion mechanism of the invention is installed in a charged particle device, especially a transmission electron microscope. Substantially the same effects can also be obtained in a scanning electron microscope and focused-ion-beam machining device including a sample micromotion mechanism similar to that of the invention. 
     First Embodiment 
     The configuration, functions, and advantageous effects of a first embodiment of the present invention are described below referring to  FIGS. 1 to 11 . 
       FIG. 1  is a cross-sectional view of a transmission electron microscope having a sample micromotion mechanism according to the first embodiment of the present invention. A housing  123  is connected to a vacuum pump  119  via a vacuum tube  121  and includes a high-vacuum region thereinside. In the housing  123 , an aligning coil  103  adjusts a direction in which an electron beam emitted from a charged particle source  101  is to travel, and then an aperture  105  adjusts a spread of the electron beam. After these adjustments, a focusing lens  107  adjusts a direction of the electron beam so that the beam focuses upon a very small region of a sample (not shown) that is supported by a sample micromotion mechanism  201  in a magnetic field which an objective lens  109  generates. After being transmitted through the sample, the electron beam is enlarged by the magnetic field that the objective lens  109  has generated. The electron beam is further enlarged by a projection lens  111  that follows the objective lens  109 , and projected on a fluorescent screen  113 . A pattern image of the electron beam which has been converted into visible light on the fluorescent screen  113  is visually observed from an observation hole  115  and/or recorded with a camera (not shown). Alternatively, the fluorescent screen  113  is directed away from an irradiation path of the electron beam via a hinge  125 , then the pattern image of the electron beam is converted into visible light using any other kind of detector, for example a scintillator, and this visible light is recorded with a CCD camera  117 . 
       FIG. 2  is a sectional view of the sample micromotion mechanism according to the first embodiment of the present invention, the sectional view representing a section in the direction that the electron beam is transmitted through the sample. 
     A sample holder  203  holds the sample  200  at its distal end, and places the sample  200  in the magnetic field generated by the objective lens  109 . The sample holder  203 , supported by O-rings  207  inside a sliding cylinder  205 , moves in an x-direction by means of a micromotion screw  209  and a spring  211 , thereby positioning the sample. 
     While internally retaining the sample holder  203 , the sliding cylinder  205  moves by leverage with a sliding section as a pivot point, the sliding section including a spherical fulcrum  213  and a spherical seat  215 . In addition, the sliding cylinder  205  moves in a y-direction by means of a micromotion screw  217  and a spring  219 , thereby positioning the sample  200 . Furthermore, while internally retaining the sample holder  203 , the sliding cylinder  205  moves in a z-direction by means of a micromotion screw (not shown) and a spring (not shown) by the leverage motion with the above sliding section as the pivot point, and thereby positions the sample  200 . 
     While internally retaining the sliding cylinder  205 , an entire rotary cylinder  221  rotates in a theta (θ) direction via bearings  225  placed between the rotary cylinder  221  and the objective lens  109 , and thereby positions the sample. 
     Strain Measuring Elements 
       FIG. 3  shows a method of detecting deformation of the sliding cylinder  205  that occurs near the spherical fulcrum  213  for positioning the sample  200  in the y-direction. In the present invention, the deformation of the sliding cylinder  205  that causes a sample drift is detected by means of detecting strain that occurs in constituent members of the sample micromotion mechanism. When the sample holder  203  positions, for example, in the y-direction the sample  200  mounted at the distal end of the holder, the sample holder  203  moves by leverage with the sliding section as a pivot point, the sliding section including the spherical fulcrum  213  and spherical seat  215  located near a distal end of the sliding cylinder  205 . During the leverage movement of the sample holder  203 , friction against the sliding section, gravity that the spherical fulcrum  213  exerts upon the spherical seat  215 , and a force that acts with the above friction to pull the spherical fulcrum  213  into an internal vacuum of the housing  123  generate a frictional force that impedes the movement of the sliding cylinder  205 , thereby cause a bending moment, and consequently deform the sliding cylinder  205 . 
     During the positioning of the sample  200  in the y-direction, the deformation of the sliding cylinder  205  becomes a maximum when the sliding cylinder  205  begins to move causing static friction to a sliding surface. When the sliding cylinder  205  is moving, the static friction changes into kinetic friction slightly smaller than the static frictional force, and when the sliding cylinder  205  comes to rest, the kinetic friction changes back into static friction, thus maximizing the deformation. At this time, repulsion of the sliding cylinder  205  due to the deformation is balanced with the bending moment caused by the static friction. 
     If external vibration is transmitted to the sliding section, although the static friction against the sliding section remains invariant, the gravity that the spherical fulcrum  213  exerts upon the spherical seat  215 , or the force that pulls the spherical fulcrum  213  into the internal vacuum of the housing  123  may decrease with the vibration and correspondingly reduce the frictional force. If the frictional force is thus reduced, the repulsion that the sliding cylinder  205  develops will decrease to a level at which the repulsion becomes balanced with the frictional force, and the decrease will phase down the deformation of the sliding cylinder  205  and cause the sample drift. 
     The present invention features reducing the deformation of the sliding cylinder  205  for suppressed sample drift, with attention focused upon the fact that even after the static frictional force has decreased for a reason such as the external vibration, the sample drift does not occur if the repulsion of the sliding cylinder  205  due to the deformation is smaller than the frictional force. 
     If the static frictional force deforms the sliding cylinder  205 , strain occurs as calculated from a rate of the deformation and an elastic modulus of the members subjected to the deformation. Accordingly the strain that has occurred in the sliding cylinder  205  is detected in the present invention. 
     The bending moment, caused by the static frictional force that deforms the sliding cylinder  205 , is maximized in a vicinity of a boundary  301  between a sliding surface of the spherical fulcrum  213  and a body surface of the sliding cylinder  205 . Strain sensors  303  and  305  for detecting strain are placed in a corresponding section.  FIG. 4  represents a method of using a metal resistance strain sensor as an element for detecting strain in the sliding cylinder  205 . 
     While it is efficient to place the strain sensor at where the bending moment is maximized, the placement location for the strain sensor is not always limited to there. For strain detection in y- and z-directions, in particular, it is efficient to place the strain sensor at where the bending moment is maximized; while for strain detection in x-direction, any other suitable placement location may be selected. 
     The objective lens  109  and the sample micromotion mechanism  201  placed adjacently thereto are shown in sectional view in  FIG. 4  for illustrative purposes. When the sample  200  is positioned in the y-direction, the micromotion screw  217  is operated to cause the sliding cylinder  205  to move by leverage with the spherical fulcrum  213  as its center. This leverage motion deforms the sliding cylinder  205  and generates strain proportional to the amount of deformation. After the sliding cylinder  205  has moved in the y-direction, a metal resistance strain sensor detects the strain as the strain sensor  305  placed in the vicinity of the boundary between the spherical fulcrum  213  and the body surface of the sliding cylinder  205 , and sends the strain value to a resistance meter  307 . The resistance meter  307  then detects and indicates a resistance value corresponding to the strain value. 
     The metal resistance strain sensor is placed on the body surface of the sliding cylinder  205  by, for example, bonding this sensor with an epoxy-based, phenol-epoxy-based, or cyanoacrylate-based adhesive, or any other suitable adhesive. For example, if the resistance value detected without strain on the metal resistance strain sensor is 120Ω and this resistance value changes to 120.00Ω after the movement of the sliding cylinder  205  in the y-direction, the change in the resistance value indicates that strain equivalent to a longitudinal deformation of 52 nm per 10 mm in a vicinity of the strain sensor  305  is occurring in the sliding cylinder  205 . If the frictional force occurring between the spherical fulcrum  213  and the sliding surface of the spherical seat  215  by reason of external vibration or the like decreases and the deformation decreases by 10%, this means that the sample  200  drifts by 5 nm in the y-direction. 
     Accordingly, the micromotion screw  217  is operated to reduce the deformation of the sliding cylinder  205  so that the resistance value detected will be 120Ω, the resistance value detected without strain on the metal resistance strain sensor. Operating the micromotion screw  217  in this way will only reduce the deformation of the sliding cylinder  205  and will not move the sample  200  in the y-direction. When the micromotion screw  217  is operated so that the resistance meter  307  indicates a resistance value of 120.008Ω, strain equivalent to a longitudinal deformation of 42 nm per 10 mm in the vicinity of the strain sensor  305  will only be occurring and the sample drift due to external vibration or the like will not occur. 
     Although a conventional method of detecting deformation in terms of a change in distance needs a distance to be used as a reference, the method of detecting strain in accordance with the present invention does not need the reference distance affected by measurement accuracy. This feature allows deformation to be detected with high accuracy and hence the sample drift to be reduced accurately. In addition, since the reference distance is unnecessary, in a sample micromotion mechanism called the side-entry type enabling it to be reduced in size and usually requiring placement in such a form that the sample micromotion mechanism crosses a magnetic field generated by an objective lens, the objective lens can be easily placed in the sample micromotion mechanism reduced in size so as to reduce an area of the magnetic field which the objective lens will block for a narrower spatial distribution of the magnetic field relative to an enlarging scale factor. 
     Wheatstone Bridge Circuit 
       FIG. 5  shows a method of detecting strain due to the deformation of the sliding cylinder  205  and representing the strain value detected. This method employs a Wheatstone bridge circuit and other elements. The Wheatstone bridge circuit  311 , used instead of the resistance meter  307 , is formed by combining three resistors,  309 , each of which has the same value as a resistance value of a metal resistance strain sensor, or a strain sensor  305 . The strain sensor  305  connects to the Wheatstone bridge circuit  311 . A direct-current (DC) power supply  313  also connects to the Wheatstone bridge circuit  311 . 
     The Wheatstone bridge circuit  311  has its output signal value represented as a signal level indication on a signal level indicator  317  via a signal amplifier  315 . When the sliding cylinder  205  suffers no deformation, the values of the resistors which constitute the Wheatstone bridge circuit  311  including the metal resistance strain sensor, or the strain sensor  305 , all become the same and the signal level indicator  317  indicates zero. 
     With the Wheatstone bridge circuit  311 , even if the DC power supply as a reference voltage device which applies a voltage to the Wheatstone bridge circuit  311 , and the signal amplifier as a differential amplifier which amplifies the output signal from the Wheatstone bridge circuit  311  become unstable, the output from the differential amplifier is held at zero when strain is absent. 
     When the sliding cylinder  205  suffers deformation, the resistance value of the metal resistance strain sensor which is the strain sensor  305  differs from the values of the resistors constituting the Wheatstone bridge circuit  311 . In this case, the signal level indicator  317  indicates a deviation corresponding to the deformation. Accordingly the micromotion screw  217  (see  FIG. 4 ) is operated to obtain a zero deviation indication on the signal level indicator  317 . This reduces the deformation of the sliding cylinder  205 , thus enabling the occurrence of any sample drift, caused by external vibration or the like, to be suppressed. 
     Using the Wheatstone bridge circuit  311  to detect the resistance value of the metal resistance strain sensor which is the strain sensor  305 , therefore, allows presence/absence of strain to be detected sensitively and hence any sample drift to be minimized. 
       FIG. 6  shows another method of detecting strain due to the deformation of the sliding cylinder  205  and representing the strain value detected. 
     The strain in the sliding cylinder  205  due to the deformation caused thereto when the sample  200  is positioned in a positive y-direction is detected using a Wheatstone bridge circuit  311  formed by combining three kinds of elements: a metal resistance strain sensor as a strain sensor  305 , which detects deformation of the strain sensor itself in the same direction as that at which the sensor elongates; a metal resistance strain sensor as a strain sensor  303 , which detects deformation of the strain sensor itself in the same direction as that at which the sensor contracts; and two resistors,  309 , having the same resistance values as those of the above two strain sensors. 
     This detection method uses the two metal resistance strain sensors placed at different positions. This method, therefore, generates an output of the Wheatstone bridge circuit  311  that is twice as large as in the detection method described per  FIG. 5 , so that the presence/absence of strain can be detected more sensitively and thus any sample drift can be minimized. 
       FIG. 7  shows a method of detecting deformation of a sample holder  203  when a sample  200  is positioned in a positive x-direction, and then reducing the deformation so as to reduce a drift of the sample. For ease in description, a sliding cylinder  205  internally holding the sample holder  203  is omitted from the figure. 
     When the sample holder  203  is positioned in the x-direction by actions of a micromotion screw  209  and a spring  211 , O-rings  207  arranged between the sliding cylinder  205  and the sample holder  203  so as to maintain the internal vacuum of the housing  123  slide while suffering deformation. 
     A repulsive force of the O-rings  207  due to the deformation of their own is exerted upon the sample holder  203 , and thus the sample holder  203  also deforms. Since the deformation of the O-rings  207  progressively decreases with external vibration and the like, the resulting repulsion of the O-rings  207  also decreases, which in turn reduces the deformation of the sample holder  203  and generates a sample drift in the x-direction. The deformation of the sample holder  203  becomes a maximum between the spring  211  exerting a force upon the sample holder  203 , and an O-ring  353 , the O-ring that is closest to the spring  211 , of all O-rings that generate repulsion of the spring  211 . 
     For this reason, a strain sensor  351  is placed at where the deformation of the sample holder  203  is maximized, and the deformation of the sample holder  203  is detected there. 
     A Wheatstone bridge circuit  311  is formed by combining three resistors,  309 , each of which has the same value as a resistance value of a metal resistance strain sensor, or a strain sensor  351 . The strain sensor  351  connects to the Wheatstone bridge circuit  311 . A direct-current (DC) power supply  313  also connects to the Wheatstone bridge circuit  311 . The Wheatstone bridge circuit  311  has its output signal value represented as a signal level indication on a signal level indicator  317  via a signal amplifier  315 . The signal level indicator  317  indicates zero when the sliding cylinder  205  suffers no deformation, that is, under the state where values of the resistors which constitute the Wheatstone bridge circuit  311  including the metal resistance strain sensor, or the strain sensor  351 , all become the same. When the sample holder  203  suffers deformation and the resistance value of the metal resistance strain sensor which is the strain sensor  351  differs from the values of the resistors constituting the Wheatstone bridge circuit  311 , the signal level indicator  317  indicates a deviation corresponding to the deformation. Accordingly the micromotion screw  209  is operated to obtain a zero deviation indication on the signal level indicator  317 . This reduces the deformation of the sample holder  203 , thus enabling the occurrence of any sample drift, caused by external vibration or the like, to be suppressed. Using the Wheatstone bridge circuit  311  to detect the resistance value of the metal resistance strain sensor which is the strain sensor  351 , therefore, allows the presence/absence of strain to be detected sensitively and hence any sample drift to be minimized. 
     Because the strain caused by the deformation of the sample micromotion mechanism  201  is smaller than one microstrain, use of a semiconductor resistance strain sensor more accurate than metal resistance strain sensors allows the deformation of the micromotion mechanism  201  to be reduced by detecting very small strain, thereby suppressing a drift of the sample. 
     Semiconductor Resistance Strain Sensor 
       FIG. 8  shows an example of using a semiconductor resistance strain sensor to detect strain. This semiconductor resistance strain sensor replaces one of the strain sensors shown as  303 ,  305 , and  351  in  FIGS. 3, 4, 5 , and  6 . Two resistance strain sensors that detect strain, and two resistors that exhibit the same resistance values as those of the resistance strain sensors are used to form a Wheatstone bridge circuit on one single-crystal silicon substrate. The Wheatstone bridge circuit operates as the semiconductor resistance strain sensor. 
     The strain sensor elements and resistors needed for the Wheatstone bridge circuit to detect strain are all arranged on one single-crystal silicon substrate. Hence, this circuit has a feature that since changes in ambient temperature give a uniform influence to the circuit, the strain value detected will be practically insusceptible to the changes in ambient temperature. 
     A change rate of resistance and that of elongation/contraction are represented as follows by expression (1):
 
Δ R/R =(1+2σ)Δ L/L   (1)
 
(where R: resistance value, ΔR: change in resistance value, L: length of the strain sensor, ΔL: change in the length of the strain sensor, and σ: Poisson&#39;s ratio of the material.)
 
     For a metallic material, σ is about 0.3, so it follows that the change rate of resistance is 1.6 times that of elongation/contraction. 
     A change in resistance value due to the elongation/contraction of the semiconductor material is represented as follows by expression (2):
 
Δ R/R =(π E+ 1+2σ)Δ L/L   (2)
 
(where π: piezoresistance coefficient of the material, and E: elastic modulus of the material.)
 
     If the semiconductor material is a silicon semiconductor, forming a conductor by diffusing a p-type impurity over a plane of crystal orientation (100) of the material gives a piezoresistance coefficient of 72×10 −11  Pa −1  and an elastic modulus of 185×10 9  Pa of the silicon, so the change rate of resistance becomes 133 times that of elongation/contraction. 
     Hence, sensitivity increases above that of the strain sensor which detects the change in resistance value due to the elongation/contraction of the metallic material. 
     A number denoted by three numerals in parentheses in  FIG. 8  signifies a Miller index, and numbers each denoted by three numerals in angle brackets in the figure signify orientations in terms of vectors, with the first digit denoting a component of the x-direction, the second digit denoting a component of the y-direction, and the third digit denoting a component of the z-direction, and with a negative directional component being expressed with a bar above the numeral. 
     The single-crystal silicon substrate  401  in  FIG. 8  has a crystal plane expressed by miller index (001). A rectangle with length in direction &lt;110&gt; is patterned on the substrate  401  and then p-type impurities are selectively diffused over the rectangle to form resistance strain sensors  403  and  405 . In addition, a V-shaped region with length in direction &lt;100&gt; is patterned on the substrate  401  and then p-type impurities are selectively diffused over the region to form V-shaped resistors  407  and  409 . The resistance strain sensors  403  and  405  and the V-shaped resistors  407  and  409  are further combined to form a Wheatstone bridge circuit that operates as the semiconductor resistance strain sensor. 
     In this method, forming the strain sensor, on which the region obtained by diffusing the p-type impurity in the silicon substrate is elongated in direction &lt;110&gt;, provides an effect in that sensitivity can be made even higher by further increasing the region in length. Additionally, forming the resistors into a V-shape at the region obtained by diffusing p-type impurity layers in the substrate, and in such a form that a linear section forming the V-shape has length in direction &lt;100&gt;, provides effects in that no strain detection error is caused by changes in atmospheric pressure that are exerted upon the sample micromotion mechanism, and thus in that a detection error due to deformation whose direction is orthogonal to that of the deformation of the sample micromotion mechanism can be suppressed. Furthermore, using the same p-type impurity layer to construct the resistance strain sensors and the resistors provides an effect in that since respective resistance values and temperature dependence of these resistance values can be controlled to substantially the same value between the resistance strain sensors and the resistors, no strain detection error is caused by changes in a temperature of the sample micromotion mechanism. 
     Since the Wheatstone bridge circuit  411  is formed on one silicon substrate,  401 , connecting a DC power supply between terminals  413  and  417  causes the resistance values of the resistance strain sensors  403  and  405  to change in response to the deformation of the sample micromotion mechanism  201 , and develops an output of a voltage corresponding to the deformation of the sample micromotion mechanism  201 . This output then has its value indicated on the signal level indicator  317  (see  FIG. 6 or 7 ) via the signal amplifier  315  (also see  FIG. 6 or 7 ). Accordingly the micromotion screw  209  or  217  (see  FIG. 4 ) is operated to obtain a zero deviation indication on the signal level indicator  317 . This reduces the deformation of the sample micromotion mechanism  201 , thus enabling the occurrence of any sample drift, caused by external vibration or the like, to be suppressed. 
       FIG. 9  shows another example of a semiconductor resistance strain sensor for detecting strain. Two resistance strain sensors,  421  and  423 , that detect strain, and two resistors,  425  and  427 , that exhibit the same values as those of the resistance strain sensors are constructed on a single-crystal silicon substrate  401  to form a Wheatstone bridge circuit that operates as the semiconductor resistance strain sensor. 
     The resistance strain sensors  421  and  423  are formed by patterning a rectangle with length in direction &lt;110&gt; on a crystal plane expressed by Miller index (001), and then selectively diffusing n-type impurities over the rectangle. The resistors  425  and  427  are likewise formed by patterning a rectangle with length in direction &lt;110&gt; and then selectively diffusing n-type impurities over the rectangle. The resistance strain sensors  421  and  423  and the resistors  425  and  427  are further combined to form the Wheatstone bridge circuit that operates as the semiconductor resistance strain sensor. 
     In this method, since the Wheatstone bridge circuit  411  is likewise formed on one silicon substrate,  401 , connecting a DC power supply between terminals  413  and  417  causes resistance values of the resistance strain sensors  423  and  425  to change in response to the deformation of the sample micromotion mechanism  201 , and causes a voltage commensurate with the deformation of the sample micromotion mechanism  201  to be output between terminals  415  and  419 . This output then has its value indicated on the signal level indicator  317  (see  FIG. 6 or 7 ) via the signal amplifier  315  (also see  FIG. 6 or 7 ). Accordingly the micromotion screw  209  or  217  (see  FIG. 4 ) is operated to obtain a zero deviation indication on the signal level indicator  317 . This reduces the deformation of the sample micromotion mechanism  201 , thus enabling the occurrence of any sample drift, caused by external vibration or the like, to be suppressed. 
       FIG. 10  shows yet another example of a semiconductor resistance strain sensor for detecting strain. Two resistance strain sensors,  429  and  431 , that detect strain, and two resistors,  433  and  435 , that exhibit the same values as those of the resistance strain sensors are constructed on a single-crystal silicon substrate  401  to form a Wheatstone bridge circuit that operates as the semiconductor resistance strain sensor. 
     The resistance strain sensors  429  and  431  are formed by patterning a rectangle with length in direction &lt;110&gt; on a crystal plane expressed by Miller index (001), and then selectively diffusing p-type impurities over the rectangle. The resistors  433  and  435  are likewise formed by patterning a rectangle with length in direction &lt;110&gt; and then selectively diffusing n-type impurities over the rectangle. The resistance strain sensors  429  and  431  and the resistors  433  and  435  are further combined to form the Wheatstone bridge circuit that operates as the semiconductor resistance strain sensor. 
     This method features high sensitivity since whereas the p-type impurity diffusion layer exhibits a change in resistance of a positive value in response to a change in positive strain in a strain measuring direction, the n-type impurity diffusion layer exhibits a change in resistance of a negative value in response to the change in the positive strain in the strain measuring direction. 
     In this method, since the Wheatstone bridge circuit  411  is likewise formed on one silicon substrate,  401 , connecting a DC power supply between terminals  413  and  417  causes resistance values of the resistance strain sensors  429  and  431  to change in response to the deformation of the sample micromotion mechanism  201 , and causes a voltage commensurate with the deformation of the sample micromotion mechanism  201  to be output between terminals  415  and  419 . 
     This output then has its value indicated on the signal level indicator  317  (see  FIG. 6 or 7 ) via the signal amplifier  315  (also see  FIG. 6 or 7 ). Accordingly the micromotion screw  209  or  217  (see  FIG. 4 ) is operated to obtain a zero deviation indication on the signal level indicator  317 . This reduces the deformation of the sample micromotion mechanism  201 , thus enabling the occurrence of any sample drift, caused by external vibration or the like, to be suppressed. 
     Sample Drift Adjusting Sequence 
       FIG. 11  is a flowchart that describes a sequence for detecting strain in the sample micromotion mechanism  201  so as to suppress the sample drift described per  FIGS. 1 to 10 , and then reducing deformation of the sample micromotion mechanism  201  by conducting a micromotion adjustment upon the sample micromotion mechanism  201  according to the detected strain. This sequence includes: 
     (1) Executing “OPERATE SAMPLE MICROMOTION MECHANISM” (step  1101 ) to select a position at which a sample is to be irradiated with an electron beam; 
     (2) Executing “STOP SAMPLE MICROMOTION MECHANISM” (step  1102 ) to stop the sample micromotion mechanism after the selection of the irradiating position; 
     (3) After step  1102 , executing “SHIELD ELECTRON BEAM” (step  1103 ) by closing the aperture  105  to intercept the electron beam midway so as to prevent radiation from causing the strain sensor to generate noise when the electron beam hits a part of the sample or housing and generates the radiation; 
     (4) After step  1103 , executing “SLIGHTLY ROTATE ROTARY CYLINDER  221 ” (step  1104 ) by operating a rotating handle  223  so as to rotate the rotary cylinder  221  of the sample micromotion mechanism  201  to a rotating position at which the that minimum deformation of the sliding cylinder  205  corresponds to a minimum strain, the execution of step  1104  being thus beneficial for removing any twist and strain caused to the sliding cylinder  205  by the rotation of the rotary cylinder  221 , and reducing a magnitude of a strain detection error due to the deformation of the sliding cylinder  205 ; 
     (5) Determining whether the strain in the sliding cylinder  205  due to the deformation is the minimum (step  1105 ); if the determination results in ‘yes’ (affirmative), control proceeds to step  1106 , and if the determination results in ‘no’ (negative), control is returned to step  1104 ; 
     (6) After one of steps  1104 ,  1105 , and  1006 , slightly moving the sliding cylinder  205  of the sample micromotion mechanism  201  in the y- or z-direction to the position at which the minimum deformation of the sliding cylinder  205  corresponds to the minimum strain (step  1106 ); 
     (7) Determining whether the strain in the sliding cylinder  205  due to the deformation is the minimum (step  1107 ); if the determination results in ‘yes’ (affirmative), control proceeds to step  1109 , and if the determination results in ‘no’ (negative), control is returned to step  1106 ; 
     (8) After one of steps  1106 ,  1107 , and  1109 , slightly moving the sample holder  203  of the sample micromotion mechanism  201  in the x-direction to a position at which the that minimum deformation of the sample holder  203  corresponds to a minimum strain (step  1108 ); 
     (9) Determining whether the strain in the sliding cylinder  205  due to the deformation is the minimum (step  1109 ); if the determination results in ‘yes’ (affirmative), control proceeds to step  1110 , and if the determination results in ‘no’ (negative), control is returned to step  1108 ; and 
     (10) Finally, opening the aperture  105  to execute “REMOVE ELECTRON BEAM SHIELD” (step  1110 ). 
     In accordance with the above sequence, strain is detected in the direction that the entire micromotion mechanism rotates. Next after the sample micromotion mechanism has been slightly moved in the rotating direction according to the detected strain value, the sample micromotion mechanism is slightly moved in a twisting direction according to a strain value detected in the direction that the entire micromotion mechanism twists and deforms. In the sample-positioning structure constituting the sample micromotion mechanism  201 , deformation of a larger member disposed on the outside is reduced first and then that of a member disposed internally to the larger member is reduced, so that the deformation of the sample micromotion mechanism  201  can be reduced without repetitive adjustment. This makes it possible to suppress the occurrence of the sample drift. 
     In addition, the strain value that has been detected in the rotating direction is increased to, for example, 10 times an initial value, the strain value that has been detected in the twisting direction is held intact, after respective absolute values have been calculated a value obtained by adding all the calculated values is presented. According to the presented value, the sample micromotion mechanism is slightly moved in automatic operation mode in both the rotating direction and the twisting direction. Thus the strain in the rotating direction of the sample micromotion mechanism can be reduced with priority over the strain in the twisting direction, and this enables the deformation of the sample micromotion mechanism to be reduced accordingly. This makes it possible to reduce the sample drift accurately. 
     Second Embodiment 
     A configuration as well as functions and advantageous effects of a second embodiment of the present invention are described below referring to  FIGS. 12 to 15 . 
     The present embodiment presents an example of suppressing a sample drift under automatic control. The first embodiment employs micromotion screws to adjust the sample drift, whereas the second embodiment employs micromotion motors instead of the micromotion screws. The micromotion motor has its operation controlled by signals transmitted from a control unit not shown. 
     The following describes details of the control. 
       FIG. 12  shows a sample micromotion mechanism  500  in sectional view for illustrative purposes. A micromotion motor  501  moves a sample holder  203  in an x-direction. A micromotion motor  503  moves a sliding cylinder  205  in a y-direction. A micromotion motor  505  moves the sliding cylinder  205  in a z-direction. A micromotion motor  507  rotates a gear  509  in a theta (θ) direction, the gear  509  being placed on a rotary cylinder  221  as shown. The sample holder  203  includes a strain sensor  351  (see  FIG. 7 ) that detects strain due to deformation in the x-direction, the sliding cylinder  205  includes a strain sensor  303  or  305  (see  FIG. 6 ) that detects strain due to deformation in the y-direction, and the sliding cylinder  205  includes a strain sensor  511  or  513  that detects strain due to deformation in the z-direction. 
     First while making the micromotion motor  507  operate to rotate the rotary cylinder  221  in a direction opposite to a direction in which the rotary cylinder last rotated, the sample micromotion mechanism  500  records any changes in y-direction and z-direction strain values, detected with respect to the deformation of the sliding cylinder  205 . For example, such results as shown in  FIG. 13  are obtained from recorded data. Referring to  FIG. 13 , changes in strain values (arbitrary scales) are plotted on a vertical axis and changes in a control variable (angle of rotation in the θ-direction) of the micromotion motor  507  are plotted on a horizontal axis, with its left end denoting a rotational angle of zero degrees and with the rotational angle increasing as it goes rightward. 
     Next while making the micromotion motor  503  operate to move the sliding cylinder  205  in a direction opposite to the y-direction in which the sliding cylinder last moved, the sample micromotion mechanism  500  records any changes in y-direction strain value, detected with respect to the deformation of the sliding cylinder  205 . For example, such a result as indicated by curve (a) in  FIG. 14  is obtained from recorded data. Referring to  FIG. 14 , changes in strain value (arbitrary scale) are plotted on a vertical axis and changes in a control variable (amount of movement in the y- or z-direction) of the micromotion motor  503  or  505  are plotted on a horizontal axis, with its left end denoting a zero amount of movement and with the amount of movement increasing as it goes rightward. 
     Next while making the micromotion motor  505  operate to move the sliding cylinder  205  in a direction opposite to the z-direction in which the sliding cylinder last moved, the sample micromotion mechanism  500  records any changes in z-direction strain value, detected with respect to the deformation of the sliding cylinder  205 . For example, such a result as indicated by curve (b) in  FIG. 14  is obtained from recorded data. 
     Furthermore, while making the micromotion motor  501  operate to move the sample holder  203  in a direction opposite to the x-direction in which the sample holder last moved, the sample micromotion mechanism  500  records any changes in x-direction strain value, detected with respect to the deformation of the sample holder  203 . For example, such a result as shown in  FIG. 15  is obtained from recorded data. Referring to  FIG. 15 , changes in strain value (arbitrary scale) are plotted on a vertical axis and changes in a control variable (amount of movement in the x-direction) of the micromotion motor  501  are plotted on a horizontal axis, with its left end denoting a zero amount of movement and with the amount of movement increasing as it goes rightward. 
     Furthermore, the sample micromotion mechanism  500  calculates a mean square of (z-direction strain) and (y-direction strain) from the results shown in  FIG. 13 , and without moving the rotary cylinder  221 , drives the micromotion motor  507  to the position corresponding to the control variable for the micromotion motor  507  that minimizes the mean square as per the least squares method. 
     Furthermore, in accordance with the results shown in  FIG. 14 , without moving the sliding cylinder  205 , the sample micromotion mechanism  500  drives the micromotion motors  503  and  505  to the positions corresponding to the control variables for the micromotion motors  503  and  505  that minimize respective absolute values of curves (a) and (b). 
     Furthermore, in accordance with the result shown in  FIG. 15 , without moving the sample holder  203 , the sample micromotion mechanism  500  drives the micromotion motor  501  to the position corresponding to the control variable for the micromotion motor  501  that minimizes an absolute value of the strain in the sample holder  203 . 
     The flowchart shown in  FIG. 11  is programmed and saved as the above drift-adjusting sequence in an internal storage section of the control unit. In this automated drift-adjusting sequence, initial data and measured data are also saved in the storage section (not shown). 
     In this scheme, operations for minimizing the deformation of the sample micromotion mechanism  500  are not performed through a manual trial-and-error process. Instead, they can be automated. This makes it unnecessary for an operator to perform an operation for suppressing the sample drift, and enables the operator to observe the sample efficiently after sample positioning has been completed by the sample micromotion mechanism  500 . 
     If the above sequence for minimizing the deformation of the sample micromotion mechanism  500  is repeated twice, a force that deforms the sample micromotion mechanism  500  can be exchanged at minimum load between the rotary cylinder  221 , the sliding cylinder  205 , and the sample holder  203 , and the sample micromotion mechanism  500  having further less deformation can therefore be provided.