Patent Publication Number: US-2007108159-A1

Title: Probe for scanning probe microscope and method of producing the same

Description:
TECHNICAL FIELD  
      The present invention relates to a structure of a probe for a scanning probe microscope including a small cantilever capable of detecting a displacement and a velocity from a back surface of a substrate by an optical means, and also relates to a method for fabricating the probe.  
     BACKGROUND ART  
       FIG. 1  is a perspective view of the structure of a probe for a known scanning probe microscope, where  FIG. 1 (A) is a perspective view illustrating the structure of a known probe for a scanning probe microscope according to a first embodiment and  FIG. 1 (B) is a perspective view illustrating the structure of a known probe for a scanning probe microscope according to a second embodiment.  
      As shown in  FIG. 1 (A), the probe includes a single beam cantilever  102  extending from a base (substrate)  101 . A probe tip  103  suitable for an object to be measured and a measurement method is attached to the top end of the cantilever  102  as needed. In general, a material for the base  101  is silicon. Typically, the base  101  has a width of about 1.6 millimeters and a length of about 3.4 millimeters. The cantilever  102  is formed from a variety of materials, such as silicon, silicon nitride, or those covered with an evaporated metal. As shown in  FIG. 1 (B), a beam cantilever  104  having a variety of shapes, such as a triangular shape, is applied as needed. Typically, the cantilever  102  or  104  has a length of 100 micrometers to several 100 micrometers.  
       FIGS. 2 and 3  illustrate how the probe for a known scanning probe microscope is typically used.  
      In the drawings, a base  111  is mounted to a scanning apparatus (not shown) including a piezoelectric device. A probe tip  113  of a cantilever  112  scans a surface of an object to be measured  114  such that the cantilever  112  traces the surface. The scanning probe microscope detects the deformation of the cantilever  112  caused by interaction between the probe tip  113  and the object to be measured  114 , such as an atomic force or a magnetic force, so that the topography or magnetization of the object to be measured  114  may be visualized using computer graphics. In general, the scanning probe microscope detects the deformation of the cantilever  112  by optical means.  
      As shown in  FIG. 2 , in the case of employing the optical means including an optical lever, a laser beam  115  is reflected off the back surface of the cantilever  112  and the angle of a reflected beam  116  is detected by a photo diode. In contrast, as shown in  FIG. 3 , in the case of employing the optical means including an optical interferometer, an incident beam  122  and an output beam  123  travel along the same optical path.  
      In either case, in order to prevent a beam reflected off the back surface of the cantilever  112  from being blocked by an end  111 A of the base  111 , the cantilever  112  protrudes outwards from the base  111  instead of being located on the base  111 .  
      The known probes are discussed in the following patent documents 1 to 4:  
      Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-66127 (pages 4 to 5 and FIG. 1),  
      Patent Document 2: Japanese Unexamined Patent Application Publication No. 9-105755 (pages 4 to 5 and FIG. 1),  
      Patent Document 3: Japanese Unexamined Patent Application Publication No. 10-90287 (pages 3 to 4 and FIG. 1), and  
      Patent Document 4: Japanese Unexamined Patent Application Publication No. 10-221354 (pages 3 to 5 and FIG. 1).  
     DISCLOSURE OF INVENTION  
      However, the structure of these known probes causes a problem when the cantilever becomes small.  
      In an operating mode known as a non-contact mode of a scanning probe microscope used for finding a force between an object to be measured and the cantilever from a change in the natural frequency of the cantilever, reducing the size of a cantilever facilitates the speed-up of measurement and the detection of a small force.  
       FIG. 4  is a perspective view of a known probe, in which only the dimensions of a cantilever is reduced.  
      As shown in  FIG. 4 , since a base  131  is used to attach a probe onto a main body of a microscope, the dimensions of the base  131  remain substantially constant regardless of the dimensions of the cantilever (an oscillator). As described above, each side of the base  131  ranges from 1 millimeter to a few millimeters. By contrast, if the length of a miniaturized cantilever  132  is, for example, 10 micrometers and if the degree of parallelization between the base  131  and an object to be measured  133  is not precisely controlled, a corner  134  or  135  of a front edge of the base  131  are brought into contact with the object to be measured  133  before the miniaturized cantilever  132  reaches the object  133 .  
      In addition, since most part of the object to be measured  133  is hidden by the base  131  and cannot be observed, it is difficult to determine the position with which the miniaturized cantilever  132  is to be brought into contact.  
      Accordingly, it is an object of the present invention to provide a probe for a scanning probe microscope and a method of fabricating the probe capable of accurately measuring an object without the base of a cantilever being brought into contact with the object to be measured and without the object being hidden by the base of the cantilever.  
      To achieve the above-described object, the present invention is characterized in that:  
      (1) A probe for a scanning probe microscope includes a base of the probe for the scanning probe microscope, a support cantilever extending horizontally from the base, and a measuring cantilever which is disposed on the top end of the support cantilever and which has a length less than or equal to 20 micrometers and a thickness less than or equal to 1 micrometer;  
      (2) In the probe for a scanning probe microscope described in (1), the base and the support cantilever are formed from single-crystal silicon, the measuring cantilever is formed from a single-crystal silicon thin film, and the measuring cantilever is coupled with the top end of the support cantilever;  
      (3) In the probe for a scanning probe microscope described in (1), the top end of the support cantilever is processed to have a sloped surface so that the top end of the support cantilever does not prevent the measuring cantilever from being optically observed;  
      (4) In the probe for a scanning probe microscope described in (1), the thickness of the measuring cantilever is less than the thickness of the coupling portion between the measuring cantilever and the support cantilever so that the length of the measuring cantilever is precisely determined;  
      (5) In the probe for a scanning probe microscope described in (1), the width of the measuring cantilever is less than the width of the coupling portion between the measuring cantilever and the support cantilever so that the length of the measuring cantilever is precisely determined;  
      (6) In a method for fabricating the probe for a scanning probe microscope described in (2), the base and the support cantilever are formed by processing a single-crystal silicon substrate, the measuring cantilever is formed by processing a single-crystal silicon thin film of an SOI substrate different from the single-crystal silicon substrate, the support cantilever is bonded with the measuring cantilever, and a handling wafer and a buried oxide film are removed from the SOI substrate;  
      (7) In the method for fabricating the probe for a scanning probe microscope described in (6), a probe tip is formed at the top end of the measuring cantilever by means of wet etching. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of the structure of a probe for a known scanning probe microscope;  
       FIG. 2  illustrates how the probe for the known scanning probe microscope is typically used (in a first case);  
       FIG. 3  illustrates how the probe for the known scanning probe microscope is typically used (in a second case);  
       FIG. 4  is a perspective view of a known probe, in which only the dimensions of a cantilever are reduced;  
       FIG. 5  is a perspective view of a probe for a scanning probe microscope defined by claim  1  of the present invention;  
       FIG. 6  is a perspective view of a probe for a scanning probe microscope defined by claim  3  of the present invention;  
       FIG. 7  is a perspective view of the top end of a support cantilever of a probe defined by claim  4  of the present invention;  
       FIG. 8  is a perspective view of the top end of a support cantilever of a probe defined by claim  5  of the present invention;  
       FIG. 9  illustrates an example of a fabrication process of a base of a probe and a support cantilever of a scanning probe microscope according to the present invention;  
       FIG. 10  illustrates an example of a fabrication process of a measuring cantilever according to the present invention;  
       FIG. 11  illustrates a fabrication process of a support cantilever and a measuring cantilever according to the present invention;  
       FIG. 12  illustrates a probe fabricated using a method defined by claim  7  of the present invention; and  
       FIG. 13  illustrates a fabrication process of a probe fabricated using the method defined by claim  7  of the present invention. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      The present invention can provide the following advantages:  
      (A) A probe having a structure in which a miniaturized measuring cantilever is provided to the top end of a support cantilever, as disclosed in claim  1 , facilitates observation of an object to be measured and efficiently prevents a base from being brought into contact with the object;  
      (B) A probe including a base and a support cantilever formed from a single-crystal silicon and a measuring cantilever formed from a single-crystal silicon thin film, as disclosed in claim  2 , can provide a high Q value of vibration, in particular, in a non-contact mode AFM in which the measuring cantilever is vibrated;  
      (C) A probe having a sloped top end of a support cantilever, as disclosed in claim  3 , can prevent the top end of the support cantilever from blocking a light beam when a measuring cantilever is optically observed;  
      (D) A probe in which the length of a measuring cantilever is determined by the length of a portion that is thinner, as disclosed in claim  4 , can precisely set the length of the measuring cantilever regardless of the alignment precision between the measuring cantilever and a support cantilever;  
      (E) A probe in which the length of a measuring cantilever is determined by the length of a portion whose width is decreased, as disclosed in claim  5 , can precisely set the length of the measuring cantilever regardless of the alignment precision between the measuring cantilever and a support cantilever;  
      (F) A method for fabricating a probe in which a support cantilever and a measuring cantilever are fabricated from different substrates and subsequently are coupled with each other, as disclosed in claim  6 , facilitates a process to form a complicated shape on the substrates as compared with a method in which a probe is fabricated without using a coupling process, thereby providing a high manufacturing yield;  
      (G) A method for fabricating a probe tip in which the probe tip is fabricated by means of wet etching, as disclosed in claim  7 , provides a high consistency with the fabrication method disclosed in claim  6  and can provide a probe tip having a small curvature radius by using the crystal anisotropy regardless of the precision of a lithographic process.  
      The present invention provides a probe for a scanning probe microscope including a base of the probe, a support cantilever extending horizontally from the base, and a measuring cantilever which is disposed on the top end of the support cantilever and which has a length less than or equal to 20 micrometers and a thickness less than or equal to 1 micrometer.  
     First Embodiment  
      Embodiments of the present invention are now herein described in detail.  
       FIG. 5  is a perspective view of a probe for a probe microscope described in claim  1  of the present invention, where  FIG. 5 (A) is a perspective view of an entire probe for a probe microscope and  FIG. 5 (B) is an enlarged view of the top end of a support cantilever of the probe.  
      As shown in these drawings, a support cantilever  2  extends from a base  1 . A measuring cantilever  3  is mounted on the top end of the support cantilever  2 . A probe tip  4  is provided on the top end of the measuring cantilever  3  as needed.  
      A probe as defined in claim  2  includes the base  1  and the support cantilever  2  formed from single-crystal silicon and the measuring cantilever  3  formed from a single-crystal silicon thin film.  
      This structure can provide a high Q value of vibration of a measuring cantilever in a non-contact mode AFM (atomic force microscopy) in which the measuring cantilever is used while being vibrated.  
       FIG. 6  is a perspective view of a probe for a probe microscope defined in claim  3  of the present invention, where  FIG. 6 (A) is a perspective view of an entire probe for a probe microscope and  FIG. 6 (B) is an enlarged view of the top end of a support cantilever of the probe.  
      As shown in these drawings, a support cantilever  12  extends from a base  11 . In order to prevent a light beam from being blocked by the support cantilever  12  when a measuring cantilever  13  coupled with the top end of the support cantilever  12  is optically observed, a sloped surface  12 A is formed on the top end of the support cantilever  12 . Additionally, a slope angle θ of the sloped surface  12 A is an acute angle.  
       FIG. 7  is a perspective view of the top end of a support cantilever of a probe defined by claim  4  of the present invention, where  FIG. 7 (A) is a perspective view of a first embodiment in which a measuring cantilever has a triangular shape and  FIG. 7 (B) is a perspective view of a second embodiment in which a measuring cantilever has a rectangular shape.  
      In  FIG. 7 (A), reference numeral  21  denotes a support cantilever and reference numeral  22  denotes a measuring cantilever. The measuring cantilever  22  forms a flat triangular shape as a whole. Reference numeral  23  denotes a base portion of the measuring cantilever  22 , reference numeral  24  denotes the front portion of the measuring cantilever  22 , reference numeral  25  denotes a stepped portion of the measuring cantilever  22  in the thickness direction, and reference numeral  26  denotes a probe tip. Here, provided are a base (not shown) of the probe for a probe microscope, the support cantilever  21  extending horizontally from the base, and the measuring cantilever  22  which is coupled with the top end of the support cantilever  21  and which has a length less than or equal to 20 micrometers and a thickness of less than or equal to 1 micrometer.  
      The front portion  24  of the measuring cantilever  22  functions as a measuring unit whose deformation is observed when scanning an object. The stepped portion  25  is formed at the border between the base portion  23  and the front portion  24  in the thickness direction so that the thickness of the front portion  24  is less than that of the base portion  23 . Here, L 1  represents the set length of the front portion  24  of the measuring cantilever  22 .  
      In  FIG. 7 (B), reference numeral  31  denotes a support cantilever and reference numeral  32  denotes a measuring cantilever. The measuring cantilever  32  forms a flat rectangular shape as a whole. Reference numeral  33  denotes a base portion of the measuring cantilever  32 , reference numeral  34  denotes the front portion of the measuring cantilever  32 , reference numeral  35  denotes a stepped portion of the measuring cantilever  32  in the thickness direction, and reference numeral  36  denotes a probe tip. As in  FIG. 7 (A), a base of the support cantilever  31  is not shown here.  
      The front portion  34  of the measuring cantilever  32  functions as a measuring unit whose deformation is observed when scanning an object. The stepped portion  35  is formed at the border between the base portion  33  and the front portion  34  in the thickness direction so that the thickness of the front portion  34  is less than that of the base portion  33 . Here, L 2  represents the set length of the front portion  34  of the measuring cantilever  32 .  
      In the probes having such structures, the length of the measuring cantilever can be defined as a length of the portion having a thinner thickness. Accordingly, the length of the measuring cantilever can be precisely set regardless of the alignment precision between the measuring cantilever and the support cantilever.  
       FIG. 8  is a perspective view of the top end of a support cantilever of a probe defined by claim  5  of the present invention, where  FIG. 8 (A) is a perspective view of a first embodiment in which a front portion of a measuring cantilever has a triangular shape and  FIG. 8 (B) is a perspective view of a second embodiment in which a front portion of a measuring cantilever has a rectangular shape.  
      In  FIG. 8 (A), reference numeral  41  denotes a support cantilever and reference numeral  42  denotes a measuring cantilever. Reference numeral  43  denotes a base portion of the measuring cantilever  42  and reference numeral  44  denotes the front portion of the measuring cantilever  42 . The front portion  44  has a flat triangular shape having a sharp top end. Reference numeral  45  denotes a stepped portion in the width direction formed at a border between the base portion  43  and the front portion  44 . Reference numeral  46  denotes a probe tip. A base of the support cantilever  41  is also not shown here.  
      Here, provided are a base (not shown) of the probe for a probe microscope, the support cantilever  41  extending horizontally from the base, and the measuring cantilever  42  which is coupled with the top end of the support cantilever  41  and which has a length less than or equal to 20 micrometers and a thickness of less than or equal to 1 micrometer.  
      The front portion  44  of the measuring cantilever  42  functions as a measuring unit whose deformation is observed when scanning an object. The stepped portion  45  is formed at the border between the base portion  43  and the front portion  44  in the width direction so that the width of the front portion  44  is less than that of the base portion  43 . Here, L 3  represents the set length of the front portion  44  of the measuring cantilever  42 .  
      In  FIG. 8 (B), reference numeral  51  denotes a support cantilever and reference numeral  52  denotes a measuring cantilever. Reference numeral  53  denotes a base portion of the measuring cantilever  52  and reference numeral  54  denotes the front portion of the measuring cantilever  52 . The front portion  54  has a flat rectangular shape. Reference numeral  55  denotes a stepped portion in the width direction. Reference numeral  56  denotes a probe tip. A base of the support cantilever  51  is also not shown here.  
      Here, provided are a base (not shown) of the probe for a probe microscope, the support cantilever  51  extending horizontally from the base, and the measuring cantilever  52  which is coupled with the top end of the support cantilever  51  and which has a length less than or equal to 20 micrometers and a thickness of less than or equal to 1 micrometer.  
      The front portion  54  of the measuring cantilever  52  functions as a measuring unit whose deformation is observed when scanning an object. The stepped portion  55  is formed at the border between the base portion  53  and the front portion  54  in the width direction so that the width of the front portion  54  is less than that of the base portion  53 . Here, L 4  represents the set length of the front portion  54  of the measuring cantilever  52 .  
      In the probes having such structures, the length of the measuring cantilever can be defined as a length of the portion having a short width. Accordingly, the length of the measuring cantilever can be precisely set regardless of the alignment precision between the measuring cantilever and the support cantilever.  
      In particular, for the probes shown in  FIGS. 7 and 8 , when the measuring cantilever is coupled with the support cantilever, the strict alignment precision is not required. Thus, these probes are effective for this case.  
      A method for fabricating a probe defined by claim  6  is now herein described with reference to  FIGS. 9, 10 , and  11 .  
       FIG. 9  illustrates an example of a fabrication process of a base of a probe and a support cantilever of a probe microscope according to the present invention, where  FIG. 9 (A) is an overall perspective view,  FIG. 9 (B) is an enlarged view of an area A shown in  FIG. 9 (A), and  FIG. 9 (C) is an enlarged view of an area B shown in  FIG. 9 (B).  
      Here, a base  63  and a support cantilever  64  are fabricated while being supported by a frame  62  formed by processing a single-crystal silicon substrate  61 . Although, in  FIG. 9 , a plurality of the bases  63  is supported by the frame  62  of the single-crystal silicon substrate  61 , the numbers of the bases  63  and the support cantilevers  64  processed at a time and the manner for supporting the bases  63  and the support cantilevers  64  are not limited to those shown in  FIG. 9 .  
       FIG. 10  illustrates an example of a fabrication process of a measuring cantilever according to the present invention, where  FIG. 10 (A) is an overall perspective view,  FIG. 10 (B) is an enlarged view of an area A shown in  FIG. 10 (A).  
      As shown in  FIG. 10 , a measuring cantilever  76  is fabricated by processing a single-crystal silicon thin film  75  of an SOI substrate  71 . Here, the measuring cantilever  76  has a triangular shape. However, the shape of the measuring cantilever is not limited to a triangular shape. In  FIG. 10 (B), reference numerals  74  and  73  denote a buried oxide film of the SOI substrate  71  and a handling wafer, respectively.  
       FIG. 11  illustrates a fabrication process of a support cantilever and a measuring cantilever according to the present invention, where  FIG. 11 (A) illustrates a coupling process of the support cantilever and the measuring cantilever and  FIG. 11 (B) is an enlarged view of the top end of a fabricated probe.  
      The SOI substrate  71  in which the measuring cantilever  76  shown in  FIG. 10  is formed is turned over (not shown in  FIG. 10 ). Thereafter, the SOI substrate  71  is bonded with the silicon substrate  61  in which the base  63  and the support cantilever  64  shown in  FIG. 9  are formed.  
      As a result, as shown in  FIG. 11 (A), the measuring cantilever  76  is coupled with the top end of the support cantilever  64 .  
      Subsequently, the handling wafer  73  and the buried oxide film  74  of the SOI substrate  71  are removed, and a probe is fabricated.  FIG. 11 (B) is an enlarged view of a top end of the support cantilever of the fabricated probe.  
       FIG. 12  illustrates a probe fabricated using a method defined by claim  7  of the present invention, where  FIG. 12 (A) is a perspective view of a top end of the support cantilever of the probe and  FIG. 12 (B) is a back perspective view of the top end of the support cantilever of the probe.  FIG. 13  illustrates a fabrication process of a probe using the method defined by claim  7  of the present invention, where  FIG. 13 (A) illustrates the top end of the support cantilever  64  shown in  FIG. 11 (B) viewed from the back and FIGS.  13 (B)-(D) illustrate a fabrication process of a probe tip  79  by enlarging the top end of the measuring cantilever  76 .  
      In these drawings, reference numeral  64  denotes a support cantilever, reference numeral  76  denotes a measuring cantilever, reference numeral  77  denotes a silicon oxide film or a silicon nitride film, reference numeral  78  denotes a sloped surface, and reference numeral  79  denotes a probe tip.  
      Here, the surface orientation of the measuring cantilever  76  must be a surface ( 100 ). Also, the longitudinal axis of the measuring cantilever  76  must be oriented towards an orientation &lt; 110 &gt;. As shown in  FIG. 13 (B), the side surface and the back surface of the measuring cantilever  76  are covered by a silicon oxide film or silicon nitride film  77 . However, the top surface of the measuring cantilever  76  must not be covered by the silicon oxide film or silicon nitride film  77 . This silicon oxide film or a silicon nitride film  77  can be formed in a variety of ways. For example, in a stage shown in  FIG. 11 (A), a nitride film is formed over the entire surface. If a chemical that does not dissolve the nitride film is used when the handling wafer  73  and the buried oxide film  74  of SOI substrate  71  are removed, the side surface and the back surface of the measuring cantilever  76  are covered by the silicon nitride film  77  in a stage shown in  FIG. 11 (B) without any further processing.  
      Subsequently, in a stage shown in  FIG. 13 (C), the measuring cantilever  76  is wet-etched by an alkaline aqueous solution so that the thickness of the measuring cantilever  76  is reduced. The sloped surface  78  with a front edge is formed in a surface ( 111 ) because of slow etching speed therein. Finally, in a stage shown in  FIG. 13 (D), the silicon oxide film or silicon nitride film  77  is removed so as to achieve the probe tip  79 .  
      Although the invention has been shown and described with reference to the foregoing embodiments, various modifications may be made without departing from the spirit and scope of the invention and these modifications should not be excluded from the spirit and scope of the invention.  
     INDUSTRIAL APPLICABILITY  
      According to the present invention, a probe for a scanning probe microscope is provided that can precisely measure the deformation of a cantilever caused by interaction between a probe tip and an object to be measured, such as an atomic force or a magnetic force, and that can provide a fine and precise measurement.