Abstract:
A scanning probe microscope is capable of not only avoiding contact between a sample and a probe but also capable of shortening the time from detection of the sample during scanning by the probe to raising the probe to the top surface of the sample. A Z-axis controlling amount calculating mechanism controls a displacement amount of a cantilever or an oscillation amount in oscillating the cantilever so as to be constant, and a Z-axis driving mechanism drives in a Z direction the cantilever or a sample based on the controlling signal from the Z-axis controlling amount calculating mechanism. A driving range limiting device limits a driving range of the Z-axis driving mechanism, and a driving range setting device optionally sets the driving range so that the probe is lowered to a set position even in an area with no sample.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a scanning probe microscope in which force exerted between a cantilever provided on its top end with a minute probe and a sample is used for measuring an uneven shape and/or a physical characteristic of a surface of the sample, particularly, a scanning probe microscope in which contact between a sample and a probe can be prevented.  
         [0003]     2. Background Art  
         [0004]     A scanning probe microscope comprises a cantilever having a minute probe on a top end thereof, displacement detecting means for detecting displacement of the cantilever, Z-axis controlling amount calculating means for calculating a controlling amount for keeping constant a displacement amount of the cantilever or an oscillation amount in oscillating the cantilever, Z-axis driving means for driving in a Z direction the cantilever or a sample on the basis of the controlling amount from the Z-axis controlling amount calculating means, XY scanning means for relatively moving the probe in a direction of an XY plane with respect to the sample and storing means for storing the controlling amount outputted from the Z-axis controlling amount calculating means and a signal outputted from the displacement detecting means. In the scanning probe microscope, the probe scans a surface of the sample by means of the XY scanning means so as to measure an uneven shape and/or a physical characteristic of the surface of the sample. An atomic force microscope (AFM) for detecting atomic force such as Van der Waals force exerted between a top end of a probe and a surface of a sample is known as an example of the above.  
         [0005]     The atomic force microscope includes a Z-axis controlling means for controlling a distance between a probe and a sample so that displacement of the cantilever would be kept constant. The atomic force microscope is a scanning probe microscope in which the Z-axis controlling amount is converted into an image so as to convert an uneven shape of a surface of the sample into a three-dimensional image.  
         [0006]     When an end of a small piece sample such as a semiconductor is measured in a scanning probe microscope, in the case that there is an area where no sample is in an area, the Z-axis driving means are lowered in the area with no sample to a lower limit position in a driving range for the purpose of detecting a nonexistent sample, so that the probe is vainly moved in the air for scanning. Scanning under such a condition is likely to cause breakage of the probe due to contact between the probe and the sample when a scanning speed is faster than a moving speed of the probe to a top surface of the sample after the probe detects the sample although the Z-axis driving means are retracted in order to raise the probe to the top surface of the sample when the probe detects a side surface of the sample.  
         [0007]     Therefore, in order to avoid the contact between the probe and the sample, used are a method for scanning at a speed low enough for the probe to safely move to the top surface of the sample or a method for observing a height of the probe in scanning so as to stop scanning once to move the probe to a safe position before restarting scanning in the case of exceeding a previously set value of limit in height of the probe (refer to Patent Reference 1, for example).  
         [0008]     The above-mentioned methods, however, have the following problems.  
         [0009]     1) It takes excess time for the measurement, so that the throughput is lowered.  
         [0010]     2) Scanning is stopped once in the case of using a piezo electric element or the like as a scanning device, so that a nonlinear characteristic of the piezo electric element causes distortion of an image.  
         [0000]     [Patent Reference 1] JP-A-2003-166927 (Paragraphs 0025-0029, FIG. 4)  
       SUMMARY OF THE INVENTION  
       [0011]     In view of the above-mentioned conventional problems, an object of the invention is to provide a scanning probe microscope not only capable of avoiding the contact between a probe and a sample but also capable of shortening time from detection of the sample by means of the probe in scanning to raise of the probe to the top surface of the sample.  
         [0012]     For the purpose of solving the above-mentioned problems, the invention is a scanning probe microscope comprising: a cantilever having a minute probe on a top end thereof; displacement detecting means for detecting displacement of the cantilever; Z-axis controlling amount calculating means for calculating a controlling amount for keeping constant a displacement amount of the cantilever or an oscillation amount in oscillating the cantilever; Z-axis driving means for driving in a Z direction the cantilever or a sample on the basis of the controlling amount from the Z-axis controlling amount calculating means; XY scanning means for relatively moving the probe in a direction of an XY plane with respect to the sample and storing means for storing the controlling amount outputted from the Z-axis controlling amount calculating means and a signal outputted from the displacement detecting means, wherein the probe scans a surface of the sample by means of the XY scanning means so as to measure an uneven shape and/or a physical characteristic of the surface of the sample, the scanning probe microscope further comprising: controlling range limiting means for limiting a driving range of the Z-axis driving means; and controlling range setting means for optionally setting the driving range.  
         [0013]     Further, the invention is a scanning probe microscope comprising: inclination measuring means for measuring inclination in the Z-axis direction of the sample; and Z-axis controlling range calculating means for calculating a Z-axis controlling range necessary for measurement on the basis of the inclination of the surface of the sample, which is calculated by means of the inclination measuring means, wherein the driving range calculated by means of the Z-axis controlling range calculating means is set in the Z-axis controlling range setting means.  
         [0014]     As described above, in invention, means for limiting the driving range of the Z-axis driving means are arranged to be included, so that the probe is only lowered to a set position even in scanning an area with no sample. Accordingly, even when an end of a sample is detected in the area with no sample, a distance that the probe is moved for raise to the top surface of the sample can be shortened, and thereby, shortened is the time that the probe is moved to the top surface of the sample. This can prevent the probe from being in contact with the sample, so that an effect of protecting the probe can be achieved.  
         [0015]     Further, the scanning speed is not necessary to be slowed for the purpose of avoiding contact between the probe and the sample as in the conventional case. This allows an effect of carrying out efficient measurement to be achieved.  
         [0016]     Moreover, a creep phenomenon that large drive of the Z-axis driving means in the Z direction influences the subsequent operation in the Z direction of the Z-axis driving means to be changed causes distortion of the measured data in the Z direction usually. The invention, however, has also an effect of preventing such a bad influence. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a block diagram showing a structure of a scanning probe microscope in a first embodiment of the invention.  
         [0018]      FIG. 2  illustrates traces of a probe in scanning in the invention.  
         [0019]      FIG. 3  is a flowchart of an operation of the scanning probe microscope in the first embodiment of the invention.  
         [0020]      FIG. 4  is a block diagram showing a structure of a scanning probe microscope in a second embodiment of the invention.  
         [0021]      FIG. 5  illustrates a sample, which is measured in the scanning probe microscope in the second embodiment of the invention.  FIG. 5 ( a ) is a top view of the sample.  FIG. 5 ( b ) is a lateral view of the sample.  
         [0022]      FIG. 6  is a flowchart of an operation of the scanning probe microscope in the second embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     An area with no sample is originally not necessary, so that a probe is not required to be lowered to a lower limit position.  
         [0024]     Accordingly, in the invention, means for limiting a driving range in the Z-axis to a preset value are provided to compare a Z-axis target position calculated by means of the Z-axis controlling amount calculating means with a Z-axis driving lower limit value set in advance. Z-axis control is performed as it is when the Z-axis target position is located upper than the Z-axis driving lower limit value. On the other hand, when the Z-axis target position is located lower then the Z-axis driving lower limit value, a Z-axis controlling position is replaced with the lower limit value so that the probe would not be lowered more than the set lower limit value even in the case of no sample in a place to be scanned by the probe.  
         [0025]     In accordance with the invention, as shown in  FIG. 1 , in a scanning probe microscope comprising: a cantilever  3  having a minute probe  4  on its top end; displacement detector  6  for detecting displacement of the cantilever  3 ; Z-axis controlling amount calculating means  12  for calculating a controlling amount for controlling a displacement amount of the cantilever or an oscillation amount in oscillating the cantilever to be fixed; Z controlling range judging means  13  for judging the controlling amount from the Z-axis controlling amount calculating means  12 ; Z driving signal generating means  14  for generating a driving signal on the basis of the controlling amount from the Z-axis controlling amount calculating means  12 ; scanning signal amplifying means  15  for amplifying a controlling signal from the Z driving signal generating means  14 ; a piezo electric element  1  for driving in a Z direction the cantilever  3  on the basis of the signal amplified by means of the scanning signal amplifying means  15 ; XY scanning means for relatively moving the probe  4  on a surface of a sample in a direction of an XY plane; and storing means for storing the controlling amount outputted from the Z driving signal generating means  14  and the signal outputted from the displacement detecting means, wherein an uneven shape and a physical characteristic of the surface of the sample are measured on the basis of the displacement amount of the cantilever in scanning the surface of the sample by means of the probe, a range within which the piezo electric element  1  can be driven in the Z direction is optionally limited to be able to prevent the probe from unnecessarily lowered too much even in scanning an area with no sample. This allows the probe to be raised to a top surface of the sample immediately after detecting the sample. Embodiments of the invention will be described in detail hereinafter on the basis of the drawings.  
       Embodiment 1  
       [0026]     A first embodiment of the scanning probe microscope in accordance with the invention will be described first on the basis of FIGS.  1  to  3 .  
         [0027]      FIG. 1  is a block diagram showing a structure of a scanning probe microscope in the first embodiment of the invention. A piezo electric element  1  is used as Z-axis driving means and XY scanning means while an electrode for driving in a Z-axis direction, which is not shown, and an electrode for XY scanning, which is also not shown, are provided on an outer surface. The piezo electric element  1  is three-dimensionally driven in accordance with a voltage applied to the respective electrodes. One piezo electric element  1  (an upper one in the drawings) is fixed to a base body  2  while a cantilever  3  is detachably fixed to the other piezo electric element  1 . The cantilever  3  has a minute cantilevered spring structure in which a probe  4  is provided on one end thereof. A laser beam from a laser beam generator  5  is radiated on a surface of the cantilever  3  on which no probe is formed. Reflected laser beam of the above is incident on a displacement detector  6 . The displacement detector  6  comprises an optically detecting electrode, which has been divided into four. Displacement of the cantilever  3  is arranged to be detectable on the basis of a voltage outputted from the four-divided electrode.  
         [0028]     A sample  8  is fixed on a sample table  7  to be arranged at a location opposing to the cantilever  3 . The sample table  7  is fixed to coarse moving means  9 . The coarse moving means  9  comprise a stepping motor not shown and a screw mechanism not shown for converting a rotating movement of the stepping motor into a straight movement. The coarse moving means  9  are arranged so that the rotating movement of the stepping motor can cause the sample  8  to be vertically moved, and thereby, contacted with the cantilever  3 .  
         [0029]     XY scanning signal generating means  10  obtain scanning area information from a controlling computer  11  to generate a scanning signal. A voltage of the scanning signal is amplified by means of the scanning signal amplifying means  15  and applied to the electrode for XY-scanning, which is formed in the piezo electric element  1 . The piezo electric element  1  to which the voltage for scanning has been applied is arranged to be driven in an X direction and a Y direction in a regular cycle or in an irregular cycle so that the probe  4  of the cantilever  3  would XY-scan the surface of the sample  8 .  
         [0030]     When the probe  4  scans the surface of the sample  8 , roughness of the surface of the sample influences the cantilever  3  to be minutely displaced (bent). The displacement amount is detected by means of the displacement detector  6  so as to be inputted to Z-axis controlling amount calculating means  12 . In the Z-axis controlling amount calculating means  12 , a controlling amount corresponding to the voltage to be applied to the piezo electric element  1  is calculated on the basis of the inputted displacement amount of the cantilever  3 . The controlling amount is inputted to Z-axis controlling range judging means  13  as Z-axis controlling range limiting means. In the Z-axis controlling range judging means  13 , it is judged whether the inputted controlling amount is within the controlling range or not. In the case that the inputted controlling amount is out of the controlling range, the inputted controlling amount is changed to a controlling range limiting value, which is inputted to Z-axis driving signal generating means  14  for limiting the Z controlling range. Z-axis controlling range setting means  16  are capable of changing a judged value by inputting to the Z-axis controlling range judging means  13  a judging reference value inputted from the controlling computer  11 . In the Z driving signal generating means  14 , a driving signal is generated on the basis of the inputted controlling amount. The generated driving signal is amplified by means of the scanning signal amplifying means  15  to be inputted to the piezo electric element  1 . The piezo electric element  1  is only driven in the Z direction for the inputted signal amount, so that the cantilever is moved vertically to control the displacement amount of the cantilever  3 .  
         [0031]     Further, a controlling signal from the Z-axis driving signal generating means  14  is stored in the frame memory  19  used as storing means. The data stored in the frame memory  19  is inputted to the controlling computer  11 , which performs an imaging process into two-dimensional or three-dimensional image.  
         [0032]     In the conventional scanning probe microscope, however, the cantilever is lowered to the lowest limit position, to which the cantilever can be moved by means of the piezo electric element  1 , when the area with no sample is scanned as shown in  FIG. 2A . There is thus a time lag after the probe detects the sample before Z-axis control is operated to retract the piezo electric element so as to raise the probe to the top surface of the sample. Accordingly, when the raise of the probe is delayed with respect to the XY scanning, the sample is likely to be in contact with the probe, which causes breakage of the probe. Further, it is necessary to stop scanning once or to scan at a low speed in order to avoid the contact.  
         [0033]     Therefore, in the invention, the Z-axis controlling range judging means  13  are arranged to be used for comparing the controlling amount from the Z-axis controlling amount calculating means  12  to limit the Z-axis controlling range.  
         [0034]      FIG. 3  is a flowchart of the first embodiment. An operation of the scanning probe microscope will be described on the basis of the flowchart.  
         [0035]     Step  30 : The controlling range, which is set by means of the Z-axis controlling range setting means  16 , is set.  
         [0036]     Step  31 : Scanning is started.  
         [0037]     Step  32 : The probe  4  is vertically moved along the uneven shape of the surface of the sample to measure the vertical movement of the probe  4  as a displacement amount of the cantilever by means of the displacement detector  6 .  
         [0038]     Step  33 : The Z-axis controlling amount calculating means  12  is used for calculating a Z-axis controlling target position on the basis of the displacement amount of the cantilever.  
         [0039]     Step  34 : The controlling target position calculated in Step  33  is compared with the Z-axis controlling range set at Step  30 .  
         [0040]     Step  35 : Setting is changed so that the controlling target position would be a lower limit position when the controlling target position is lower than the lower limit position of the controlling range.  
         [0041]     Step  36 : The controlling target position (signal) calculated in Steps  34  and  35  is amplified into the controlling voltage to be applied to the piezo electric element  1 .  
         [0042]     Step  37 : The controlling target position (signal) calculated in Steps  34  and  35  is stored in the frame memory.  
         [0043]     Step  38 : It is judged whether scanning is completed or not.  
         [0044]      FIG. 2  illustrates traces of a top end of the probe in scanning in a lateral view.  
         [0045]      FIG. 2A  illustrates traces of a top end of the probe in the case that the invention is not carried out. Scanning is performed with the probe  4  being lowered to the lower limit position of the controlling range of the piezo electric element when an area with no sample  20  is scanned. When the probe  4  detects the sample  8  in scanning, a displacement signal from the displacement detector  6  is changed to retract the piezo electric element in accordance with the Z-axis driving voltage from the Z-axis controlling means, so that the probe  4  is raised to the top surface of the sample  8 . The probe  4  is, however, likely to have a collision with the sample  8  in the case that the raising speed is slower than the scanning speed.  
         [0046]      FIG. 2B  illustrates traces of the top end of the probe in the case of carrying out the invention. The probe  4  is only lowered to a set position within the Z-axis controlling range even when the area with no sample  20  is scanned. Accordingly, the probe  4  can be raised to the top surface of the sample without any trouble when the probe  4  detects the sample  8  in scanning. This allows collision between the probe  4  and the sample  8  to be avoided.  
       Embodiment 2  
       [0047]     A second embodiment of a scanning probe microscope in accordance with the invention will be described on the basis of FIGS.  4  to  6 .  
         [0048]      FIG. 4  is a block diagram of a scanning probe microscope in a second embodiment of the invention. The scanning probe microscope in the second embodiment is characterized in that sample inclination measuring means  17  and Z-axis controlling range calculating means  18  are added to the scanning probe microscope in the first embodiment shown in  FIG. 1 . Other common components are marked with the same signs and numerals as those of  FIG. 1  and detailed description of the above is omitted.  FIG. 5A  illustrates in a top view a sample, which is measured in the scanning probe microscope in the second embodiment of the invention.  FIG. 5B  illustrates in a lateral view the sample.  
         [0049]     Generally in a scanning probe microscope, a minute difference in orthogonal angle between a surface of the sample and the Z-axis driving means influences the surface of the sample so that difference in height in the Z-axis direction at any plural points, that is, inclination of the surface of the sample would be generated even when the surface of the sample is flat, in many cases. The difference in height due to the inclination becomes larger than the difference in roughness of the surface of the sample when the scanning area becomes large. Accordingly, it is sometimes difficult to properly set the Z-axis controlling range.  
         [0050]     In view of the above, the invention has a structure in which inspection measurement for measuring inclination is carried out before measuring and the inclination of the sample is measured on the basis of the measurement data obtained by the inspection measurement so as to set a proper Z-axis controlling range.  
         [0051]     The piezo electric element  1  scans an inspection area  41 , which is a minute and flat area, before main measurement. The Z-axis controlling position (height) at respective measuring points is stored in the frame memory  19  by means of the Z-axis driving signal generating means  14 . The inspection area  41  is a flat place with no large difference in level on the surface of the sample. It is possible, for example, to measure an area, which is smaller than a measuring area  40  and which has samples all over the area, to set a flat area as an inspection area on the basis of the measurement data of the above.  
         [0052]     After completing the measurement for inspection, the measurement data stored in the frame memory  19  is inputted to the sample inclination measuring means  17 . With the sample inclination measuring means  17 , a minimum value Z min  44  and a maximum value Z max  45  of the height of the inputted measurement data are measured. An inclination degree S, which indicates inclination of a sample, is calculated in the following formula (1): 
 
 S =( Z  max− Z  min)/( D  max+ D  min)  (1) 
 
 wherein D min denotes a distance from a center of the inspection area  41  to a measured position of Z min  44  and D max denotes a distance from the center of the inspection area  41  to a measured position of Z max  45 . 
 
         [0053]     The inclination degree S calculated in the formula (1) is inputted into the Z controlling range calculating means  18 . In the Z controlling range calculating means  18 , a lowest limit position Zlimit of the Z controlling position at the end of the measuring area is calculated in the following formula: 
 
 Zlimit=Zcenter−S ·{square root}( X·X+Y·Y )− Zα   (2) 
 
 wherein X denotes a length of a side of the measuring area in the inclination degree S and the main measurement, Y denotes a length of a side orthogonal to X, Z center denotes a Z controlling position at a center part of the measuring area and Zα denotes a value as a margin of the lower limit value. 
 
         [0054]     The value Zlimit calculated in the formula (2) is inputted to the Z-axis controlling range setting means  16 , so that the Z-axis controlling range can be changed. This allows a proper Z-axis controlling range to be set in view of the inclination of the surface of the sample as described above.  
         [0055]      FIG. 6  is a flowchart of an operation of the scanning probe microscope in the second embodiment of the invention. The operation will be described hereinafter on the basis of the flowchart.  
         [0056]     Step  60 : A scanning area corresponding to the inspection area  41  is set.  
         [0057]     Step  30 : The maximum width of the Z controlling range is set.  
         [0058]     Steps  31  to  38 : Description is omitted since the operation after starting scanning is same as that of the first embodiment.  
         [0059]     Step  63 : Whether the measurement is for inspection or as a main one is judged after completing scanning.  
         [0060]     Step  62 : An inclination degree is calculated on the basis of the measured data stored in the frame memory in the case of the measurement for inspection.  
         [0061]     Step  61 : A proper Z controlling range is calculated on the basis of the inclination degree calculated in Step  62 .  
         [0062]     Step  60 : A scanning area corresponding to the main measurement is set.  
         [0063]     Step  30 : The Z controlling range calculated in Step  61  is set.  
         [0064]     Steps  31  to  38 : The main measurement is performed.  
         [0065]     Step  63 : The main measurement is judged to be completed and the operation is ended.