Abstract:
Provided is an aligning method capable of setting a sample observation unit such as an optical microscope to a probe microscope observation position at high precision. A sample having a known structure is used in advance. A surface of the sample and a shape of a cantilever provided with a probe are observed using the sample observation unit such as the optical microscope. A sample observation position and a probe position which are obtained using the sample observation unit are verified, and a relative positional relationship therebetween is recorded. Then, a first mark indicating a position of the cantilever and a second mark which is displayed in conjunction with the first mark and has the relative positional relationship with the first mark are produced to align the sample relative to the second mark.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. JP2008-197941 filed on Jul. 31, 2008, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of aligning, with a sample measurement position, a probe of a probe microscope for measuring shape information such as surface roughness or a step of a sample surface, or physical information such as a dielectric constant or viscoelasticity, and a probe microscope operated by the same. 
     2. Description of the Related Art 
     In recent years, a probe microscope such as an atomic force microscope (AFM) having atomic resolution has been expected for shape measurement to evaluate a fine shape. The atomic force microscope which is a type of probe microscope has been expected as means for observing a surface shape of a novel insulating material, and investigations thereof have been conducted since the atomic force microscope was devised by G. Binnig (inventor of scanning tunneling microscope (STM)), et al. 
     An example of a schematic system structure of the probe microscope is described with reference to  FIG. 6 . 
     A sample  51  which is an object to be measured is placed on a fine movement mechanism  52  for three-dimensionally moving the sample. The fine movement mechanism  52  includes piezoelectric elements deformed in response to applied voltages, and finely aligns the sample  51  relative to a probe  53  opposed to the sample. The probe  53  is provided at a tip end of a beam member supported at only one end, which is called a cantilever  54 . 
       FIG. 7  illustrates an example of a normal shape of the cantilever. A cantilever board  64  is provided with the cantilever  54  which is the beam member supported at only one end. The fine probe  53  is formed at the tip end of the cantilever  54  and has mainly a square pyramid shape with a height of 1 μm to 2 μm. The cantilever board  64 , the cantilever  54 , and the probe  53  are made of silicon or a silicon-based material and integrally formed using, for example, an anisotropic etching technique. 
     The cantilever board  64  including the cantilever  54  is held by a cantilever holder  55 . 
     The fine movement mechanism  52  is located on a rough movement mechanism  56  including a stage for three-dimensionally moving the sample  51  and the probe  53 . The rough movement mechanism  56  is a screw feed mechanism driven by a stepping motor. 
     A displacement detection system  57  for detecting a deformation of the cantilever based on a physical force such as an interatomic force, which the probe  53  receives from the surface of the sample, is provided on the cantilever  54  side. The displacement detection system  57  includes a semiconductor laser  58  for irradiating a rear surface of the cantilever  54  with light and a four-part photo detector  59  for detecting reflected light, and is a system called an optical lever detection system for detecting a displacement (distortion deformation) of the cantilever based on the fact that a position of the light incident on the photo detector is changed by the displacement of the cantilever  54 . 
     A signal from the photo detector  59  is sent through an amplifier  60  to a Z-axis control feedback circuit  61  for controlling a Z-axis (vertical direction) interval between the sample  51  and the probe  53  to operate the fine movement mechanism  52 , thereby controlling a Z-axis positional relationship between the probe  53  and the sample  51 . In-plane scanning between the probe  53  and the sample  51  is performed by scanning with the fine movement mechanism based on a signal from an XY-driver circuit  62 . The Z-axis control and the XY-driving are performed by a computer and a control system  63 . 
     The probe  53  provided at the tip end of the cantilever  54  is brought close to the surface of the sample  51  by the rough movement mechanism  56 . A deformation of the cantilever  54  resulting from a physical force such as an interatomic force, a magnetic force, or viscoelasticity, which the probe  53  receives from the surface of the sample  51 , is detected by the displacement detection system  57 . When the deformation becomes a predetermined deformation, it is determined that the probe  53  is aligned to a measurement region. Then, the rough movement mechanism  56  is stopped, and the Z-shaft of the fine movement mechanism  52  for relatively moving the sample  51  and the probe  53 , which is located on the sample side or the cantilever side, is controlled to maintain an interval between the probe  53  and the surface of the sample  51 . While the Z-shaft of the fine movement mechanism  52  is controlled, the rough movement mechanism  56  is driven to adjust a displacement amount of the Z-shaft of the fine movement mechanism  52 , thereby aligning the probe  53  with the surface of the sample  51 . The control is performed such that a deformation value of the cantilever  54  is maintained constant. The surface of the sample is measured during scanning using the fine movement mechanism  52 , thereby visually imaging an in-plane shape of the sample and physical properties thereof. 
     An optical microscope is normally used as means for designating a location to be measured using the probe microscope, and there are a method of observing a region between the cantilever and the sample in an oblique direction and a method of performing an overhead observation using an optical member such as a prism (see, for example, JP 3023686 B). In the case of the oblique observation, a depth state on an opposite side of the optical microscope is uncertain, and hence alignment precision is low. On the other hand, in the case of the overhead observation, the alignment is easy. In general, in the case of the overhead observation, the focus of the optical microscope is adjusted on the rear surface opposed to the surface of the cantilever, to which the probe is attached, thereby verifying a probe position. Then, the focus of the optical microscope is adjusted on the surface of the sample based on the verified position, thereby determining a position to be observed using the probe microscope. As illustrated in  FIG. 8A , assume that an optical microscope  71 , a probe  72 , and an observation object  74  of a sample  73  are aligned with an optical path  75  and a moving path  76  of an alignment unit (rough movement mechanism) for aligning the sample and the probe with each other. In such a case, the probe  72  can be aligned with the observation object  74  at a close position  77  between the probe  72  and the sample  73  without any problems, and hence the observation object to be observed using the optical microscope  71  can be observed using the probe microscope. However, unless adjustment is performed, an apparatus structural displacement occurs because the optical microscope and the rough movement mechanism are different members. As illustrated in  FIG. 8B , even in the case of the overhead observation, an error  78  occurs, and alignment precision may be low. The error may vary at a magnification related to a geometric relationship as an interval between the focal position of the optical microscope and the close position  77  becomes larger. When the probe is brought close to the sample to perform alignment, the error may become smaller. However, in this case, the optical microscope observes the rear surface of the cantilever, and hence there is no difference between the focal position for observing the cantilever and the focal position for observing the sample. Therefore, when the surface of the sample is to be observed, the shadow of the cantilever interferes therewith, whereby the surface of the sample cannot be observed. 
     A scanning region of the probe microscope is normally several tens micrometers in size. When the alignment precision is low, the position may be outside the scanning region of the probe microscope. In this case, the method is conceivable, in which the scanning region of the probe microscope is increased in size. However, in a case where a smaller object is to be observed, when the scanning region of the probe microscope is large in size, the resolution is reduced because a data acquisition interval is large. Therefore, it takes time to find the observation object. 
     An example of a combination with the optical microscope is a method of adjusting the sample observation position and the probe position using a moving mechanism such as a stage based on a known sample for each cantilever exchange in a case where the optical microscope is provided at a position different from that of the probe (see, for example, JP 2909829 B and JP 06-201372 A). There is also a method of correcting the probe position using the optical microscope provided at a position separate from that of the probe position of the probe microscope (see, for example, JP 09-203740 A and JP 04-058102 A). Such a method may have alignment precision. However, an apparatus becomes larger, and hence the method is not suitable for a small apparatus. 
     Therefore, a probe microscope capable of easily aligning a position to be observed has been desired. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in order to solve the problems described above. Therefore, an object of the present invention is to provide a probe aligning method capable of easily aligning a probe microscope observation position on a sample surface with a probe provided at a tip end of a cantilever at high precision, and a probe microscope operated by the same. 
     According to the present invention, the following means is provided in order to solve the problems described above. 
     According to a probe microscope of the present invention, a sample having a known structure is used in advance. A surface of the sample and a shape of a cantilever provided with a probe are observed using sample observation means such as an optical microscope. A sample observation position and a probe position which are obtained using the sample observation means are verified, and a relative positional relationship therebetween is recorded. Then, a first mark indicating a position of the cantilever and a second mark which is displayed in conjunction with the first mark and has the relative positional relationship with the first mark are produced to align the sample relative to the second mark. 
     The sample having the known structure is used in advance. The cantilever and sample observation positions from the observation means when the sample observation position obtained using the sample observation means and the position of the probe provided in the cantilever are verified, and the difference between the cantilever and sample observation positions from the observation means at the time of the operation of the probe microscope observation are geometrically corrected. The sample alignment is performed using mark displayed during the sample observation, to align the probe with the sample observation position. 
     The observation means for observing the surface of the sample and the shape of the cantilever, such as the optical microscope is prepared. The sample having the known structure is used in advance. The relative position between the sample observation position obtained using the sample observation means such as the optical microscope and the position of the probe provided in the cantilever is corrected to obtain the mark. When the observation object position on the sample is determined using the mark, an alignment error resulting from an apparatus structure can be reduced and the observation position can be determined with high precision using a relatively simple structure based on software operation. 
     Therefore, the observation using the optical microscope and the highly precise alignment within the probe microscope observation region can be achieved, and hence it is possible to provide a probe microscope with which the reliability of a result obtained by measurement is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a structural view illustrating an outline of a probe microscope according to a first embodiment of the present invention; 
         FIGS. 2A to 2F  are explanatory state diagrams illustrating an operation in the first embodiment of the present invention; 
         FIGS. 3A and 3B  are explanatory state diagrams illustrating an operation in a second embodiment of the present invention; 
         FIG. 4  is a structural view illustrating an outline of a probe microscope according to a fourth embodiment of the present invention; 
         FIGS. 5A and 5B  are explanatory state diagrams illustrating marking states in the fourth embodiment of the present invention; 
         FIG. 6  is a structural diagram illustrating a schematic system of a probe microscope; 
         FIG. 7  is a structural view illustrating a normal cantilever shape; and 
         FIGS. 8A and 8B  are explanatory diagrams illustrating alignment error generation states. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, a probe microscope according to a first embodiment of the present invention is described with reference to  FIGS. 1 and 2A  to  2 F. In the following embodiment, the description of fundamentally the same structure as the schematic system structure of the probe microscope as illustrated in  FIG. 6  is omitted here. 
       FIG. 1  is a structural view illustrating the probe microscope according to the first embodiment of the present invention. 
     A probe microscope unit  1  as described with reference to  FIG. 6  is provided on a vibration isolation table surface plate  2 . An optical head  3  having a function of detecting a displacement of a cantilever based on an optical lever system is provided in an upper portion of the probe microscope unit  1 . An optical microscope  4  and a CCD camera  6  are located above the optical head  3  and provided on the vibration isolation table surface plate  2  through an arm  7  and a pole  8 . An observation image showing a surface of a sample and the cantilever is sent as an optical microscope image to a computer through the CCD camera, digital-processed by the computer, and displayed on a display. Rough focal adjustment is performed by moving the arm  7  upward or downward relative to the pole  8 . Fine focal adjustment between the surface of the cantilever and the surface of the sample is performed by manual operation of a focal adjustment ring  5  of the optical microscope  4 . 
     Next, an error specific to an apparatus between an optical path of the optical microscope and a moving path of an alignment unit (rough movement mechanism) for aligning the sample and a probe with each other is calculated in the following manner. 
     1. A sample having a known structure is prepared. In this embodiment, a structural pattern which has numerals drawn at a pitch of 10 micrometers and is made of silicon is used. 
     2. The cantilever is set in the apparatus. In this embodiment, a cantilever which is provided with the probe formed at a tip end portion of the cantilever and is made of silicon is used. 
     3. The focus of the optical microscope is adjusted to the cantilever to display an optical microscope image having a relatively large observation view region. 
     4. As illustrated in  FIG. 2A , A cross mark  14  is placed as a first mark indicating a probe position in a position of a probe  13  provided at a tip end of a cantilever  12 , which is displayed on a CCD image  11  (determination of probe position). The cross mark  14  is overwritten on the CCD image  11  and thus left on the display even when the focus of the optical microscope is shifted. 
     5. The focus is adjusted to the surface of the sample. 
     6. As illustrated in  FIG. 2B , the cross mark  14  is aligned with an arbitrary observation position on a pattern sample  15 . A pattern located at the center position on the CCD image  11  is normally selected. 
     7. The probe is brought close to the surface of the sample to perform a probe microscope observation. For example, an image as illustrated in  FIG. 2C  is obtained as a result obtained by measurement. 
     8. As illustrated in  FIG. 2D , an X mark  17  is placed as a second mark on the CCD image  11  based on a result obtained by observing a pattern numeral  16  illustrated in  FIG. 2C  by the probe microscope (determination of position at which probe microscope observation is actually performed). Differences  18  and  19  between the cross mark  14  and the X mark  17  correspond to the error specific to the apparatus between the optical path of the optical microscope and the moving path of the alignment unit (rough movement mechanism) for aligning the sample and the probe with each other. A relative positional relationship is determined so as to move the cross mark  14  and the X mark  17  together and stored in a storage unit included in the apparatus (determination and storage of relative positional relationship in large observation region). 
     The operation described above is performed to complete the determination of the relative positional relationship in the large observation region, which corresponds to the error specific to the apparatus. The value is specific to the apparatus, and hence unless the structure of the apparatus is changed, it is unnecessary to perform the operation even in a case where the cantilever is exchanged for another. In the example described above, the sample having the known structure is used. A sample to be measured can be directly used. The sample having the known structure is preferably used because the observation position is easily determined. 
     Next, second and subsequent normal measurement operations are described. 
     1. A sample is set. 
     2. The cantilever is set in the apparatus. 
     3. The focus of the optical microscope is adjusted to the cantilever to display an optical microscope image. 
     4. A first mark is placed in a probe position (probe position at time of observation). As illustrated in  FIG. 2E , the cross mark  14  and the X mark  17  which is moved together therewith (position at which probe microscope observation is assumed to be actually performed, based on probe position at time of observation and stored result obtained by determination of relative positional relationship in large observation region) are put. 
     In consideration of the visibility of sample observation in the case where the two marks are displayed on a screen, whether or not the X mark  17  is displayed on the screen is switched by software operation. 
     5. The focus is adjusted to the surface of the sample. 
     6. As illustrated in  FIG. 2F , the X mark  17  is displayed. 
     The cross mark  14  and the X mark  17  are moved together, and hence different mark shapes are used in this embodiment in order to distinguish the marks from each other. Different colors may be used. When the focus or magnification of the optical microscope which is an observation unit is adjusted for sample observation, the mark for the probe position may be removed. This is more preferable because the false recognition of a mark for alignment is eliminated to perform more reliable alignment. 
     7. The X mark  17  is assumed to be located at an actual probe position, and aligned with an observation position  20  of the sample. 
     8. The probe is brought close to the surface of the sample to perform the probe microscope observation. 
     The operation as described above is performed for each cantilever exchange. When only the sample is exchanged for another, the sample observation position may be determined using the X mark  17 . 
     A second embodiment of the present invention is described with reference to  FIGS. 3A and 3B . A fundamental procedure is identical to that of the first embodiment. 
     As illustrated in  FIG. 3A , when a probe  23  is not provided at a tip end of a cantilever  22  displayed on a CCD screen  21 , a mark is imaginatively placed. This causes an error. Therefore, in the second embodiment, when the probe position is to be specified, three points  24 ,  25 , and  26  on an outline of the cantilever  22  are designated. The probe position is determined based on cantilever design information. Thus, an error caused by the designation of an operator, of the position of the probe provided not at the tip end of the cantilever but inside the cantilever can be reduced, with the result that the sample observation position can be determined. 
     In a third embodiment, when the probe position is to be specified, the outline of the cantilever is determined by image recognition software. The probe position is determined based on cantilever design information. An image recognition function is added to the system, and hence a cost thereof increases. However, the error caused by the designation of the operator can be reduced, with the result that the sample observation position can be more accurately determined. 
     In a fourth embodiment, an apparatus structure in which the present invention is used for a probe microscope for observing a large sample such as a wafer is described with reference to  FIG. 4 . 
     A unit section  31  is provided over a vibration isolation table surface plate  32 . The vibration isolation table includes a passive type or an active type. The active type is effective in a case where a low-frequency contains vibration component. The unit section  31  is located on a base  34  over the vibration isolation table surface plate  32  through elastic materials  33 . An XY-stage, which is a rough alignment mechanism  36  for aligning a sample  35  in an in-plane direction, is provided over the base  34 . A sample in-plane rotating stage  37  is provided over the rough alignment mechanism  36  to hold the sample  35  through a sample table  38 . An arm  39  is provided on the base  34 . A Z-axis stage  40 , which is a vertical direction alignment mechanism, is fixed to the arm  39 , and a fine movement mechanism  41 , which is a fine alignment mechanism, is provided thereto through the Z-axis stage  40 . The fine movement mechanism  41  includes piezoelectric elements finely distorted in response to applied voltages. A cantilever  42  is fixed to a tip end of the fine movement mechanism  41 . In the apparatus according to this embodiment, a fixing method with vacuum suction is used. The probe provided at a tip end of the cantilever  42  is aligned with a surface of the sample  35  by the Z-axis stage  40 . Distortion deformation of the cantilever  42  is detected by an optical lever mechanism (not shown) provided in the fine movement mechanism  41 , to measure the shape of the surface of the sample. In this embodiment, an optical microscope unit  43  is provided. A mirror is provided close to the cantilever  42  located at the tip end of the fine movement mechanism  41 , whereby the surfaces of the cantilever  42  and the sample  35  can be observed through the mirror. 
     The optical microscope unit  43  includes the CCD camera, a zoom mechanism for changing magnification, and a focal adjustment mechanism. The apparatus according to this embodiment has a system in which the focus and magnification of the optical microscope can be adjusted by a computer and from the outside. The focus and the magnification can be adjusted from the outside, and hence the relative positional relationship between the probe position in the large observation view region which is determined at the time of first observation and the actual probe microscope observation position can be corrected based on an error caused by a thickness of the used sample and a change of the observation view region (high-magnification observation) of the observation unit. Specifically, the X mark  17  assumed to be located at the probe position at the time of second or subsequent observation is displayed by taking geometric correction such as a proportional distribution into account using computer and external control values based on, as a reference, a value determined at the time of first observation. For example, correction relationships  44  and  45  as illustrated in  FIGS. 5A and 5B  are taken into account. Correction values are values specific to the apparatus. As in the case where the relative positional relationship between the above-mentioned probe position and the actual probe microscope observation position is determined, when the sample having the known shape is used, the correction values can be easily set.