Abstract:
This probe control method is applied to the scanning probe microscope having a probe section with a probe pointed at a sample, a detection section for detecting physical quantity between the sample and the probe, a measurement section for measuring the surface of the sample to obtain the surface information on the basis of the physical quantity when scanning the sample surface by the probe, and a movement mechanism with at least two degree of freedom. The probe control method has steps of moving the probe in a scanning direction different from the contact direction while making the probe come into contact with the sample surface, detecting the torsional state of the probe during the movement of the probe, and adjusting either or both of the rate in the scanning direction and the force in the contact direction on the basis of the detected value obtained by the detection step.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a probe control method for a scanning probe microscope, more particularly relates to a probe control method for a scanning probe microscope, that is suitable for measuring a sample surface with uneven shapes when carrying out a scanning movement of the probe along the sample surface.  
         [0003]     2. Description of the Related Art  
         [0004]     Scanning probe microscopes are known as measurement systems having measurement resolutions enabling observation of fine objects on the atomic level or size. In recent years, scanning probe microscopes have been applied to a variety of fields such as measurement of the fine profile or uneven shapes in the surfaces of wafers or substrates on which semiconductor devices are fabricated. There are various types of scanning probe microscopes for the different physical quantities for detection used for measurement. For example, there are scanning tunnel microscopes utilizing tunnel current (STM), atomic force microscopes utilizing atomic force (AFM), magnetic microscopes utilizing magnetic force (MFM), etc. The ranges of their applications have been growing as well.  
         [0005]     Atomic force microscopes are particularly suitable for detecting the fine profile or uneven shapes on sample surfaces and are proving their worth in the fields of semiconductor substrates, disks, etc. Recently, they have also been used in applications for in-line automatic inspection processes.  
         [0006]     An atomic force microscope is basically configured to be have a measurement unit operating based on the principle of atomic force microscopes. The measurement unit is provided with a tripod-type or tube-type XYZ fine actuator formed utilizing piezoelectric devices. The bottom end of the XYZ fine actuator has a cantilever with a probe at its tip. The tip of the probe is directed to the surface of the sample. The cantilever is provided with, for example, an optical lever type photo detector. In the optical lever type photo detector, a laser beam emitted from a laser light source (laser oscillator) arranged above the cantilever is reflected at the back surface of the cantilever and detected by the photo detector. If the cantilever twists or bends, the spot of the incident laser beam at the photo detector (four-divided light receiving surface, for example) changes.  
         [0007]     Therefore, if the probe and cantilever displace, it is possible to detect the direction and amount of the displacement based on a detection signal output from the photo detector. An atomic force microscope is usually further provided with a comparator and controller as a control system. The comparator compares the detection voltage signal output from the photo detector and the reference voltage and outputs an error signal. The controller generates a control signal resulting in an error signal of zero and sends this control signal to the Z-fine actuator in the XYZ fine actuator. A feedback servo control system holding the distance between the sample and probe constant is formed in this way. It is possible to use this configuration to make the probe track and scan the fine uneven shapes on the sample surface and measure their shapes.  
         [0008]     When the atomic force microscopes were first invented, the central issue was the use of their high resolution for measurement of fine shapes on the surface of dimensions on the nanometer (nm) order. At the present time, however, scanning probe microscopes have expanded in range of use to include in-line automatic inspection in the middle of in-line fabrication systems of semiconductor devices. In view of this, in actual inspection processes, it is required to measure the extremely sharp uneven shapes in the fine uneven shapes on the surfaces of the semiconductor devices fabricated on wafers.  
         [0009]     As technology for measuring the uneven surfaces, there is a scanning probe microscopes described in Japanese Patent Publication (A) No. 2002-14024. In this scanning probe microscope, when scanning the sample surface by the probe, it is controlled so that the relative velocity in a direction of the sample surface of the probe becomes fixed to the uneven surface of the sample. This control compensates for the shortage of the follow-up performance of the probe in the slope of the projected portions or the like on the sample surface.  
         [0010]     The control system under the scanning probe microscope disclosed in Japanese Patent Publication (A) No. 2002-14024 has controlled to move the probe so that the linear velocity of the probe in the direction along the inclined surface becomes fixed, paying attention to the shortage of the follow-up performance of the probe to the sample. However, a reaction force due to the sample given to the tip of the probe from the sample surface changes according to the shape of the sample and the movement directions of the probe. For this reason, the reaction force has made various twisted states in the probe in accordance with the slope of the irregularity on the sample surface, and has been the cause for producing an error to measurement data.  
         [0011]     Also, big torsion given to the probe caused serious damage to the probe, and has become the cause which shortens the life of the probe. Furthermore, for this reason, the wear amount of the probe is increased, and as a result the area that the same probe can measure is decreased. Thereby, there have arisen the problems that an accurate measurement value cannot be obtained in a large area, and that running cost is increased.  
       SUMMARY OF THE INVENTION  
       [0012]     An object of the present invention is to provide a probe control method for a scanning probe microscope, that can reduce an error of the measurement data by controlling torsional states of the probe appropriately, which are generated due to the reaction force given to the tip of the probe from the sloping sections in the uneven surface of the sample.  
         [0013]     A probe control method for a scanning probe microscope according to the present invention is configured as follows to achieve the above object.  
         [0014]     A probe control method for a scanning probe microscope is applied to the scanning probe microscope provided with a probe section with a probe arranged so as to be pointed at a sample, a detection section for detecting physical quantity between the sample and the probe, a measurement section for measuring the surface of the sample to obtain the surface information on the basis of the physical quantity when scanning the sample surface with the probe, and a movement mechanism with at least two degree of freedom. In the scanning probe microscope, the measurement section measures the surface of the sample while the movement mechanism changes the relative positional relationship between the probe and the sample by causing the probe scan the sample surface. The probe control method has steps of moving the probe in a scanning (or movement) direction different from the contact direction while making the probe come into contact with the sample surface, detecting the torsional state of the probe during the movement of the probe, and adjusting either or both of the rate (or speed) in the scanning direction and the force in the contact direction as to the probe on the basis of the detected value obtained by the detection step.  
         [0015]     In accordance with the probe control method, in case that the probe (or the cantilever) is twisted by the reaction force impressed to the tip of the probe due to the slope section of the uneven surface of the sample when performing the measurement by making the probe scan the sample surface, either or both of the rate in the scanning direction and the force in the contact direction as to the probe is adjusted to cancel the torsinal state concerning the probe, and thus the error of the measurement data can be reduced.  
         [0016]     Moreover, in the probe control method for the scanning probe microscope, preferably, the adjustment of the rate in the scanning direction of the probe is performed so that the torsional amount in the torsional state of the probe may be canceled.  
         [0017]     In the probe control method for the scanning probe microscope, preferably, the scanning direction is a probe scanning direction along the surface of the sample.  
         [0018]     In the probe control method for the scanning probe microscope, preferably, a cantilever with the probe has notches which are easy to produce the torsion.  
         [0019]     According to the present invention, in the probe movement control of the scanning probe microscope in which the probe is moved to follow the uneven shapes at measuring points on the sample surface, since the torsional state of the probe is detected during the following movement along the sample surface and further the rate in the scanning direction and/or the force in the contact direction as to the probe on the basis of the detected value, an error of the measurement data due to the torsional state of the probe can be reduced and the reliability of measurement precision can be raised, and the life of the probe can be prolonged further and thus running cost can be lowered. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     Objects and features of the present invention will become clearer from the following description of the preferred embodiments given with reference to the attached drawings, wherein:  
         [0021]      FIG. 1  is a view of the overall configuration of a measurement section and control section of a scanning probe microscope to which a probe control method of the present invention is applied;  
         [0022]      FIG. 2  is a view of illustrating a relationship among a cantilever, a probe and an optical lever type optical detection device in the scanning probe microscope;  
         [0023]      FIG. 3  is a view of illustrating an example of the scanning/measuring movement of the probe in the probe control method of the embodiment according to the present invention;  
         [0024]      FIG. 4  is a block diagram showing a control block for performing a control on a probe scanning direction in the embodiment of the probe control method;  
         [0025]      FIG. 5  is a block diagram showing a control block for performing a control on a probe contact direction in the embodiment of the probe control method;  
         [0026]      FIG. 6  is a block diagram showing a system for outputting a positional reference value; and  
         [0027]      FIG. 7  is a perspective view showing another modified example of the cantilever. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     Preferred embodiments of the present invention will be described in detail below while referring to the attached figures.  
         [0029]     The overall configuration of a scanning probe microscope (SPM) to which a probe control method according to the present invention is applied will be explained with reference to  FIG. 1 . The scanning probe microscope is an atomic force microscope (AFM) as a typical example. However, the scanning probe microscope of the present invention does not limited to the atomic force microscope.  
         [0030]     The lower part of the scanning probe microscope is provided with a sample stage  11 . The sample stage  11  has a sample  12  placed on it. The sample stage  11  is a mechanism for changing the position of the sample  12  in a three-dimensional coordinate system  13  comprised of perpendicular X-, Y-, and Z-axes. The sample stage  11  is comprised of an XY-stage  14 , a Z-stage  15  and a sample holder  16 . The sample stage  11  usually is comprised as a coarse or rough actuator causing displacement (positional change) at the sample side.  
         [0031]     The sample  12  with a relatively large area and thin shape is placed and holed on the top surface of the sample holder  16  of the sample stage  121 . The sample  12  is, for example, a substrate or wafer on the surface of which integrated circuit patterns of semiconductor devices are fabricated. The sample  12  is fixed on the sample holder  16 . The sample holder  16  is provided with a chuck mechanism for fixing the sample.  
         [0032]     In the sample stage  11 , concretely, the XY-stage  14  is a mechanism for making the sample  12  move on a horizontal plane (XY plane), while the Z-stage  15  is a mechanism for making the sample  12  move in the vertical direction (height direction). The Z-stage  15  is mounted on the XY-stage  14 . A movable distance by the sample stage  11  is hundreds of mm (millimeter) in the X or Y direction and dozens of mm (millimeter) in the Z direction.  
         [0033]     In  FIG. 1 , there are an optical microscope  18  provided with a drive mechanism  17  at a position above the sample  12 . The optical microscope  18  is supported by a drive mechanism  17 . The drive mechanism  17  is comprised of a focus-use Z-direction actuator  17   a  for moving the optical microscope  18  in the Z-axis direction and an XY direction actuator  17   b  for moving it in the XY axis directions. For mounting, the Z-direction actuator  17   a  moves the optical microscope  18  in the Z-axis direction, while the XY direction actuator  17   b  moves the unit of the optical microscope  18  and the Z-direction actuator  17   a  in the XY axis directions. The XY direction actuator  17   b  is fixed to a frame member, but in  FIG. 1 , illustration of the frame member is omitted. The optical microscope  18  is arranged with its object lens  18   a  directing the bottom and is arranged at a position approaching the surface of the sample  12  from directly above. The top end of the optical microscope  18  is additionally provided with a TV camera (imaging unit)  19 . The TV camera  19  picks up an image of a specific region of the sample surface captured by the object lens  18   a  and outputs the image data.  
         [0034]     A cantilever  21  provided with a probe  20  at its tip is arranged in a state approaching the upper side of the sample  12 . The cantilever  21  is attached to a mount  22 . The mount  22  is, for example, provided with an air suction section (not shown). The air suction part is connected to an air suction device (not shown). The cantilever  21  is fixed and attached by its large area base being attached by suction at the air suction part of the mount  22 .  
         [0035]     The above mount  22  is attached to a Z-fine actuator  23  for causing fine movement operation in the Z-direction. Further, the Z-fine actuator  23  is attached to the bottom surface of a supporting frame  25  described below for a cantilever displacement detector  24 .  
         [0036]     The cantilever displacement detector  24  is comprised of a support frame  25  to which a laser light source  26  and photo detector  27  are attached in a predetermined relative arrangement. The cantilever displacement detector  24  and the cantilever  21  are held in a constant positional relationship. A laser beam  28  emitted from the laser light source  26  is reflected at the back surface of the cantilever  21  and enters the photo detector  27 . The cantilever displacement detector forms an optical lever-type photo detector. If the cantilever  21  twists, bends, or is otherwise deformed, this optical lever-type photo detector can detect the displacement due to the deformation.  
         [0037]     The cantilever displacement detector  24  is attached to an XY fine actuator. The XY fine actuator has an X-fine actuator  29  and a Y-fine actuator  30 . The XY fine actuator makes the cantilever  21  and the probe  20  etc. move in the directions of the X-axis and Y-axis by fine distances. At this time, the cantilever displacement detector  24  is simultaneously moved. The optical position-relationship between the cantilever  21  and the cantilever displacement detector  24  does not change by canceling the displacement in the Z-axis direction using optical mirrors which are not shown in the figure.  
         [0038]     In the above explanation, the Z-fine actuator  23 , the X-fine actuator  29  and Y-fine actuator  30  usually are comprised of piezoelectric devices. The Z-fine actuator  23 , X-fine actuator  29  and Y-fine actuator  30  cause the probe  20  to displace by fine distances (for example, several angstroms (Å) to 10 μm, and maximum 100 μm) in the X-axis direction, Y-axis direction and Z-axis direction. Especially, the fine distance of the probe generated in the Z-axis direction as the displacement is dozens of μm. Further, the above Y-fine actuator  30  is attached to a frame mechanism which is not shown in the figure.  
         [0039]     In the above mounting, the observation field of the optical microscope  18  includes the surface of a specific region of the sample  12  and the tip (back surface) of the cantilever  21  including the probe  20 .  
         [0040]     Next, a control system of the scanning probe microscope will be explained. The control system is comprised of a first control device  33  and a second control device  34 . The controlling means for realizing in principle a measurement mechanism in the atomic force microscope (AFM) is constructed by programs (software) prepared in the first control device  33  as a computer. Also, the first control device  33  is for controlling the drive of a plurality of drive mechanisms. Further, the second control device  34  is positioned as a superior control device.  
         [0041]     A control means for realizing in principal the measurement section based on the atomic force microscope is constructed as follows. In force feedback signal processing section  40  and the like, a voltage signal (s 1 ) output from the photo detector  27  is inputted and compared with a reference voltage set in advance to output a deviation signal s 1 . A deviation controller within the force feedback signal processing section produces a control signal (s 2 , etc.) resulting in the deviation signal of zero and sends this control signal s 2  to the Z-fine actuator  23 , etc. The Z-fine actuator  23  receiving the control signal s 2  adjusts the height position of the cantilever  21  to hold the distance between the probe  20  and the surface of the sample  12  constant. The control loop from the photo detector  27  to the Z-fine actuator  23  is, for example, a feedback servo control loop for detecting the state of deformation of the cantilever  21  by the optical lever-type photo detector and holding the distance between the probe  20  and the sample  12  at a predetermined constant distance determined based on the above reference voltage. Due to this control loop, the probe  20  is held at a constant distance from the surface of the sample  12  in the Z-axis direction, for example. If the probe  20  scans the surface of the sample  12  in this state, it is possible to measure profile or uneven shapes of the sample surface.  
         [0042]     As to the X-fine actuator  29  and Y-fine actuator  30 , respectively, a general feedback servo-control loop is formed by using a feedback signal outputted from a displacement unit. In  FIG. 1 , a signal s 3  is an X feedback signal while it is also an X scanning command signal. Furthermore, a signal s 4  is a Y feedback signal while it is also a Y scanning command signal.  
         [0043]     Next, the first control device  33  is a control device for driving the parts of the scanning probe microscope and is provided with the following functional sections.  
         [0044]     The optical microscope  18  can be changed in position by the drive mechanism  17  comprised of the focus-use Z-direction actuator  17   a  and XY direction actuator  17   b . The first control device  33  is provided with a first drive control section  41  and second drive control section  42  for controlling the operations of the Z-direction actuator  17   a  and XY direction actuator  17   b.    
         [0045]     The image of the sample surface and cantilever  21  obtained by the optical microscope  18  is picked up by the TV camera  19  and fetched as image data. The image data obtained by the TV camera  19  of the optical microscope  18  is input to the first control device  33  and processed by an internal image processing section  43 .  
         [0046]     As to Z-fine actuator  23 , in principle, the control signal s 2  obtained through the feedback servo-control loop means the height signal of the probe  20  at the scanning probe microscope (atomic force microscope). The height signal of the probe  20 , that is, the control signal s 2 , can give information relating to the change of the height position of the probe  20 . The control signal s 2  is given to the Z-fine actuator  23  from a Z-movement control section  44  of the control device  33 .  
         [0047]     The probe  20  is made to scan the sample surface at the measurement region of the surface of the sample  12  by driving the X-fine actuator  29  and Y-fine actuator  30 . The drive of the X-fine actuator  29  is controlled by an X-movement control section  45  providing the X-fine actuator  29  with the X scan command signal and receiving the X feedback signal (S 3 ). The drive of the Y-fine actuator  30  is controlled by a Y-movement control section  49  providing the Y-fine actuator  30  with the Y scan command signal and receiving the Y feedback signal (s 4 ).  
         [0048]     Further, the XY-stage  14  and the Z-stage  15  of the sample stage  11  are controlled by an X-drive control section  46  outputting an X-direction drive signal, a Y-drive control section  47  outputting the Y-direction drive signal, and a Z-drive control section  48  outputting a Z-direction drive signal.  
         [0049]     Note that the first control device  33  is provided with a storage section (not shown) for storing setting control data, input optical microscope image data, data relating to the height position of the probe, etc. in accordance with need.  
         [0050]     The second control device  34  is positioned as the superior one for the first control device  33 . The second control device  34  performs processing such as storing and executing a usual measurement program, setting and storing usual measurement conditions, storing and executing an automatic measurement program, setting and storing its measurement conditions, storing the measurement data, performing image processing on the measurement results, and displaying the image at a display device (monitor)  35 .  
         [0051]     In particular, in case of the present invention, in the automatic measurement operation, the second control device carries out a measurement process for measuring sections of the sample surface with the slope of the uneven surface by changing the movement and posture of the probe in its movement or scanning direction. The second control device is provided with a program for performing the measurement while moving the probe in the upward direction along the slope by automatically changing the position, posture, etc. of the probe in the scanning direction.  
         [0052]     In setting the measurement conditions, basic items such as the measurement range and measurement speed, the setting up for the scanning direction of the probe, the measurement conditions, and other conditions for automatic measurement are set. These conditions are stored and managed in a setting file. Further, it is also possible to configure the microscope to have a communication function for communicating with external devices.  
         [0053]     The second control device  34  must have the above functions, so is comprised of a processing device constituted by a CPU  51  and a storage section  52 . The storage section  52  stores the above programs and condition data etc. Further, the second control device  34  is provided with an image display control section  53 , communicating section, etc. In addition, the second control device  34  is connected with an input device  36  through an interface  54 . The input device  36  can be used to set and change the measurement program, measurement conditions, data, etc. stored in the storage section  52 .  
         [0054]     The CPU  51  of the second control device  34  provides superior or higher control instructions etc. to the functional parts of the first control device  33  through a bus  55  and is provided with image data or data relating to the height position of the probe from the image processing section  43 , data processing section  44 , etc.  
         [0055]     Next, the basic operation of the above scanning probe microscope (atomic force microscope) will be explained.  
         [0056]     The tip of the probe  20  of the cantilever  21  is made to approach a predetermined region of the surface of the semiconductor substrate or other sample  12  placed on the sample stage  11 . Normally, the probe approach mechanism constituted by the Z-stage  15  is used to bring the probe  20  close to the surface of the sample  12  and atomic force is made to act to cause the cantilever  21  to bend. The bending amount due to the bending deformation of the cantilever  21  is detected by the above-mentioned optical lever-type photo detector. In this state, the probe  20  is made to move with respect to the sample surface so as to scan the sample surface (XY scan). The XY scan of the surface of the sample  12  by the probe  20  is performed by making the probe  20  move by the X-fine actuator  29  and Y-fine actuator  30  (fine movement) or by making the sample  12  move by the XY-stage  14  (coarse movement) so as to create relative movement in the XY plane between the sample  12  and the probe  20 .  
         [0057]     The probe  20  is moved by giving the X scan signal s 3  relating to X-fine movement to the X-fine actuator  29  provided with the cantilever  21  and giving the Y scan signal s 4  relating to Y-fine movement to the Y-fine actuator  30 . The scan signal s 3  relating to the X-fine movement is given from the X-movement control section  45  in the first control device  33  and the scan signal s 4  relating to the Y-fine movement is given from the Y-movement control section  49  in the first control device  33 . On the other hand, the sample is moved by giving drive signals from the X-drive control section  46  and the Y-drive control section  47  to the XY-stage  14  of the sample stage  11 .  
         [0058]     The X-fine actuator  29  or Y-fine actuator  30  is comprised of a piezoelectric device and enables high precision and high resolution scan movement. Further, the measurement range measured by the XY scan by the X-fine actuator  29  and Y-fine actuator  30  is limited by the stroke of the piezoelectric device, so becomes a range determined by a distance of about 100 μm even at the maximum. According to the XY scan by the X-fine actuators  29  and Y-fine actuator  30 , measurement in a fine, narrow range becomes possible. On the other hand, the XY-stage  14  is comprised of an electromagnetic motor as a drive, so the stroke can be enlarged up to several hundred mm. According to the XY scan by the XY-stage, measurement in a broad range becomes possible.  
         [0059]     In this way, a predetermined measurement region on the surface of the sample  12  is scanned by the probe  20  and the amount of bending (amount of deformation by bending etc.) of the cantilever  21  is controlled to become constant by the feedback servo control loop. The amount of bending of the cantilever  21  is constantly controlled to match a reference amount of bending (set by the reference voltage Vref). As a result, the distance between the probe  20  and the surface of the sample  12  is held at a constant distance. Therefore, the probe  20 , for example, moves (scans) following the fine profile or uneven shapes of the surface of the sample  12 . By obtaining the height signal of the probe, the fine profile shapes of the surface of the sample  12  can be measured.  
         [0060]     The scanning probe microscope as mentioned above, for example, is built into the middle of an in-line fabrication system for semiconductor devices (LSIs) as an automatic inspection process for inspecting the substrates (or wafers).  
         [0061]      FIG. 2  is used for explaining a principle of displacement detection by the optical lever type detection device. In the above cantilever  21  the displacements thereof are generated in either or both of “HA 1 ” and “HB 1 ” directions based on the atomic force etc. operated upon the probe at the tip. As a result, the cantilever  21  has deformation such as bending (flexure or deflection), torsion or the like. In the cantilever displacement detection section  24 , the laser light  28  emitted from the laser light source  26  is reflected at the back surface of the cantilever  21  and strikes a photodetector  27 . In  FIG. 2 , the reference numeral  27   a  designates a light receiving surface of the photodetector  27 . As an initial condition, the position of a spot which the laser light  28  strikes in the light receiving surface  27   a  of the photodetector  27  in the state with no atomic force applied to the probe  20  is memorized. Thereafter, by capturing the direction of movement of the position of the spot in the light receiving surface  27   a  of the photodetector  27 , it is possible to accurately detect the magnitude and direction of the force acting on the probe  20  through deformation of the cantilever  21 . As shown in  FIG. 2 , for example, when force of the HA 1  direction is applied to the probe  20 , the photodetector  27  catches this as a change in the spot position in the HA 2  direction. Further, when force of the HB 1  direction is applied to the probe  20 , the photodetector  27  catches this as a change in the spot position in the HB 2  direction. Here, the force in the HA 1  direction is called “torsion direction force”, while the force in the HB 1  direction is called “deflection direction force”.  
         [0062]     Note that the method for detecting the atomic force etc. applied from the sample surface to the tip of the probe  20  includes, in addition to the optical lever-type photo detector, utilization of optical interference or other optical principles or a strain detection element provided at the cantilever.  
         [0063]     Next, a characteristic probe control method in the automatic measurement method for a scanning probe microscope will be explained with reference to  FIG. 3  to  FIG. 6 . The probe control method is a method for controlling the position and posture of the probe when it goes up along the slope of the projected portions (or step portions) formed on the sample surface.  
         [0064]      FIG. 3  shows the probe control method when the probe  20  goes up along the slope of the projected portion on the surface of the sample  12 ,  FIG. 4  shows a control block for carrying out the control method on the scanning direction (movement direction) when controlling the scanning movement of the probe as shown in  FIG. 3 ,  FIG. 5  shows a control block for carrying out the control method on the contact direction when controlling the scanning of the probe as shown in  FIG. 3 , and  FIG. 6  shows a control block having a function of position reference value output processing.  
         [0065]     The probe control method explained in the present embodiment is, for example, the method of controlling the position of the probe along the upslope of the projected portion in the uneven area formed as a regular periodic uneven pattern on the surface of a wafer. Especially, it will be explained about the important method of controlling the probe position from the viewpoint of correctly measuring line edge roughness in the trenches formed on the wafer surface.  
         [0066]     In  FIG. 3 , the probe  20  is seen from the pointed end side of the cantilever  21 . The illustration for the surrounding structure related to the probe  20  and the cantilever  21  is omitted in  FIG. 3 . Moreover, the probe  20  has sufficient length required for measurement.  
         [0067]     Moreover, in  FIG. 3 , it is supposed that the probe  20  is in the position (P n ) separated from the surface of the sample  12  in an early stage, thereafter, approaches the sample surface, and further moves from the left-hand side to right-hand side in  FIG. 3  in a state of keeping it come into contact with the sample surface.  
         [0068]     Furthermore, in  FIG. 3 , a level (horizontal) section  12   a  in the surface of the sample  12  is the flat area at the lower side of the projected portion (step section) of the sample surface. The sloping section  12   b  is partially shown as a lower part of the projected portion in  FIG. 3 .  
         [0069]     Although an angle of the slope of the sloping section  12   b  is actually close to about 90 degrees, the slope shown in  FIG. 3  is drawn to have a predetermined slope angle.  
         [0070]     When the probe  20  moves to follow the surface of the sample  12 , in the transitional place from the level section  12   a  to the sloping section  12   b , the movement control method concerning the transfer (movement) direction of the probe  20  is changed by the force feedback signal under the condition that it becomes clear that the current scanning area of the probe  20  is the slope of the sloping section  12   b.    
         [0071]     That is, the probe  20  is made to approach to the sample surface and then to perform the scanning movement in touch with the sample surface when measuring a part containing the sloping section  12   b  in the surface of the sample  12  shown in  FIG. 3 . At this time, the position of the probe  20  changes with the sequence of P n , P n+1 , P n+2  and P n+3  in accordance with the movement sequence ( 1 )-( 3 ) of the probe  20  shown in  FIG. 3  (hereinafter it is called as movements ( 1 )-( 3 )). The position P n  of the probe  20  is an initial position of the probe.  
         [0072]     Next, the control procedure of movement operation of the probe  20  will be explained.  FIG. 4  to  FIG. 6  respectively shows a control block for realizing the control procedure, which is made by the second control device  34  as a device function.  
         [0073]     In the control block for scanning direction control shown in  FIG. 4 , a fine movement mechanism  401  corresponds to the above-mentioned Y-fine actuator  30  or X-fine actuator  29  assuming that the probe  20  is moved to the Y-axis or X-axis direction. The torsion force detecting section  402  corresponds to the section including the optical lever-type photo detector. The force feedback signal outputted from the torsion force detecting section  402  is to be included in the signal s 1 . A displacement detecting section  403  corresponds to an X-axis displacement detector which is not shown in  FIG. 4 . A position feedback signal outputted from the displacement detecting section  403  is to be included in the signal s 3 . In accordance with this control block, a torsion force reference value is set concerning the force feedback signal and an operation part  404  calculates the difference between the force feedback signal and the torsion force reference value. Here, the torsion direction in the probe  20  and the cantilever  21  is defined to be plus (+) value, when moving the probe in the state that the probe is pressed to contact with the sample surface. Further, in accordance with this control block, a position reference value is set concerning the position feedback signal and an operation part  405  calculates the difference between the position feedback signal and the position reference value.  
         [0074]     The position reference value given to the operation part  405  is created by the position reference value output switching section  421 . The position error signal outputted from the operation part  405  is inputted into the position reference value output switching section  421 .  
         [0075]     A signal acquired by the operation part  404  is given to an adder  408  through a converter  406 - 1 . A signal acquired by the operation part  405  is given to the adder  408  directly, or through an amplifier with a suitable gain which is not shown in the figure. The converter  406 - 1  carries out a process for setting a dead zone, and further a process for reducing the scanning rate when the probe is twisted heavier than the reference posture. The scanning command signal  409  outputted from the adder  408  is suitably supplied to the piezoelectric device  411  through a first PID controller  407  and an amplifier  410  to operate the fine movement mechanism  401 .  
         [0076]     In the control block for the contact direction control shown in  FIG. 5 , elements substantially identical to the elements explained in  FIG. 4  are respectively allotted with the same reference numeral and the detailed explanations for them are omitted. Here, the contact direction is assumed to be the Z-axis direction, and in this case the fine movement mechanism  401  is the mechanism portion of the above-mentioned Z-fine actuator. The control block includes a deflection force detecting section  412 . This deflection force detecting section  412  also comprises of the above-mentioned optical lever-type photo detector. The force feedback signal outputted from the deflection force detecting section  412  is a signal included in the signal s 1  mentioned above. The control block has an operation part  413 . The deflection force reference value is given to the operation part  43  as a reference value against to the force feedback signal outputted from the deflection force detecting section  412 . The operation part  413  calculates a difference between the deflection force reference value and the force feedback signal. The difference signal outputted from the operation part  413  is inputted into the adder  408 , and afterward is inputted into the amplifier  410  through a second PID controller  414 . A converter  406 - 2  carries out the process for setting the dead zone and a process for reducing the deflection force when the probe is twisted heavier than the reference posture and increasing it when the probe is less twisted in order to keep the posture of the probe constant.  
         [0077]     Furthermore, in regard to the control by the scanning direction control block shown in  FIG. 4  and the control by contact direction control block shown in  FIG. 5 , either or both of them are used suitably.  
         [0078]     Next, in accordance with  FIG. 6 , a process for generating the position reference value (Xd) by the position reference value output switching section  421  will be explained. This process shows an example of a method of scanning the probe with the fixed scanning rate which is obtained by reflecting the process for reducing the scanning rate of the probe by the servomechanism through the converter  406 - 1  in the calculation of the position reference value.  
         [0079]     In the system shown in  FIG. 6 , the position reference value is generated and updated every Δt as a sampling rate. This system is comprised of the position reference value output switching section  421  and a trajectory generation section  422 . The sampling rate Δt is supplied to the trajectory generation section  422  and the position reference value output switching section  421 .  
         [0080]     In the trajectory generation processing by the trajectory generation section  422 , signals of a starting position (Xs), an end position (Xe) and reference velocity (v) are inputted, and a position reference value (Xa) is calculated using these input elements. A simple system, in which the increase and decrease of the velocity is not controlled, can be expressed by the following numerical formula. 
 
 Xa ( n+ 1)= Xa ( n )+ vΔt  
 
         [0081]     Here, Xa(n) means the position reference value at the present time, and Xa(n+1) means the following position reference value at next time. In the above-mentioned trajectory generation processing, the calculation is carried out based on the numerical formula at the timing when a buffer empty signal is inputted from the position reference value output switching section  421 , and the position reference value (Xa) as the calculation result is returned to the position reference value output switching section  421 . Moreover, Xa is set to be Xe when the position reference value (Xa) exceeds the end position (Xe).  
         [0082]     Next, the position reference value output switching section  421  outputs the position reference value (Xd) in synchronizing with the signal of the sampling rate Δt. There are FIFO buffers with two or more steps in the output side of the position reference value output switching section  421 , and the input value Xa is stored in the FIFO buffers. When assuming that there are the present output value Xa(n) and the next output value Xa(n+1) in the buffers, Xd is set to be Xa(n).  
         [0083]     When judging whether the position error bx inputted has been not more than a constant value at the timing of the sampling rate Δt for the output of the position reference value (Xd) and that it has become below the constant value, the buffers are updated. That is, Xa(n) is set to be Xa(n+1).  
         [0084]     Subsequently, the position reference value output switching section  421  outputs the position reference value (Xd). Furthermore, the position reference value output switching section  421  outputs the buffer empty signal to the trajectory generation section  422  and receives the new position reference value (Xa) outputted from it.  
         [0085]     On the other hand, when judging that the position error (δx) has not become below the constant value at the timing of outputting the position target value (Xd), the buffers are not updated and the present data Xa(n) are outputted as it is.  
         [0086]     By the above, if the scanning rate of the probe were to be reduced due to the feedback of the torsion signal, after that this state is cancelled, the probe  20  can be scanned at the reference velocity (v).  
         [0087]     The movements ( 1 )-( 3 ) of the probe  20  shown in  FIG. 3  are carried out based on the control by the scanning direction control block shown in  FIG. 4 , the control by the contact direction control block shown in  FIG. 5 , and the position reference value output processing system shown in  FIG. 6 .  
         [0088]     Next, the movement of the probe  20  will be explained referring to  FIG. 3 .  
         [0089]     Movement ( 1 ): The probe  20  is caused to approach the sample  12  from the measurement start aerial position P n . This movement operation by which the probe  20  located at the upper spot separated from the sample surface is caused to approach so as to generate the atomic force between the probe and the sample surface is usually based on the operation of the Z-fine actuator  23 . The Z-movement control section  44  of the first control device  33  generates the Z-direction movement command signal concerning approaching the sample. If the probe comes into contact with the sample surface, the deflection direction force is operated on the cantilever  21 , and the reaction force fb comes to be detected. Then, the reaction force is controlled to be fixed and thereby the movement of the probe  20  is controlled to stop at the measurement start approach position P n+1 .  
         [0090]     Next, under the condition that there is no force in the torsion direction beyond the predetermined standard value, the tip position of the probe  20  is detected by use of the Z-feedback signal indicating the amount of displacement at this time. As a result the surface position of the sample can be recorded. In this case, from the point of view of forces in both of the deflection and torsion directions, it may be taken the amount of deformation of the cantilever  21  and the probe  20  into consideration  
         [0091]     Movement ( 2 ): Next, the probe  20  is caused to move in the scanning direction on the sample surface  12   a  while maintaining the state where the reaction force fb is kept to be detected. Thereby, the cantilever  21  has the torsion deformation due to frictional force ff as the probe  20  moves in the scanning direction toward the slope as shown in  FIG. 3 , and therefore the torsion force ft is detected.  
         [0092]     when the probe  20  reaches the level difference start position P n+2 , the direction of the reaction force is changed by the slope  12   b  of the projected portion or the step on the sample surface. Thereby, the torsion force in the probe  20  and the cantilever  21  increases, and the larger torsion arises in the probe  20  and the cantilever  21 .  
         [0093]     Movement ( 3 ): The probe  20  is caused to move so that it goes up along the slope  12   b  of the projected portion. In this movement along the upslope  12   b , the scanning rate of the probe  20  in the scanning direction is reduced in proportion to an increased amount for the torsion force of the probe  20 . Thereby, in the level difference middle position P n+3 , the torsion of the probe  20  is cancelled and the posture of the probe  20  returns to its former state before generating the torsion, and further it becomes possible to perform exact measurement.  
         [0094]     In the movement ( 3 ), instead of reducing the scanning rate of the probe  20 , the deflection force as the reaction force direction applied to the probe  20  may be decreased and thereby the twisted posture as to the probe  20  and the cantilever  21  can be also cancelled. Further, it may be considered that both reduction in the scanning rate and the decrease of the deflection force in the reaction force direction are performed.  
         [0095]     In the control concerning the movements ( 1 )-( 3 ), the converter  406 - 1  of the control block shown in  FIG. 4  carries out the dead zone process to the signal outputted from the operation part  404  and thereafter returns its output signal to the feedback loop for the scanning rate. By this, like the movement ( 3 ), when the state where the probe  20  was greatly twisted in its movement occurs, the scanning rate of the probe  20  can be reduced.  
         [0096]     Moreover, in case of the control block shown in  FIG. 5 , the signal outputted from the operation part  404  is returned to the feedback loop for the deflection force through the dead zone process in the converter  406 - 2 . By this, when the probe  20  is twisted greatly, the contact force can be decreased.  
         [0097]      FIG. 7  shows another cantilever  61  with two notches. The two notches  62  are formed in two long sides of the cantilever  61 , respectively. The notches  62  make the cantilever  61  to be easy to produce the torsion deformation. It is desirable to raise the detection sensitivity of the force in the torsion direction in the cantilever  61  in which the above structure capable of using the force of the torsion direction positively is adopted.  
         [0098]     The configurations, shapes, sizes, and relative arrangements explained in the above embodiments are only generally shown to an extent enabling the present invention to be understood and worked. Therefore, the present invention is not limited to the embodiments explained above and can be modified in various ways so long as not departing from the scope of the technical idea shown in the claims.  
         [0099]     The present disclosure relates to subject matter contained in Japanese Patent Application No. 2006-273929, filed on Oct. 5, 2006, the disclosure of which is expressly incorporated herein by reference in its entirety.