Abstract:
A scanning probe apparatus and a self-exciting cantilever probe therefor are provided for measuring a characteristic of a sample by scanning a lever of the probe across the sample surface. The probe has a deflectable lever extending from a base and formed integrally therewith, and a resistive body provided on the lever to excite the lever in response to a periodic bias signal applied to the resistive body. In addition, the resistive body has a resistance value that varies in response to deflection of the lever so that the resistance of the resistive body may be monitored to detect deflection of the lever.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a self-exciting and self-detecting probe suitable for use in a DFM mode in which a lever of the probe is vibrated for operation and a scanning probe apparatus utilizing the same. 
     2. Description of the Related Art 
     When a surface configuration of a sample is to be measured using a scanning probe microscope for example, a dynamic measurement mode has been widely used in which configuration data on the sample surface is obtained by causing resonance at a cantilever of the probe to tap the sample surface lightly, and controlling the position of the cantilever in a Z-direction by with servo system on a feedback basis to achieve uniform tapping. 
     Therefore, when a sample is measured in the dynamic measurement mode, configurations have been used in which a piezoelectric body, such as PZT, is coupled to a cantilever body through physical contact to excite the cantilever. 
     Measurement data has been then obtained by vibrating the cantilever with a voltage signal applied to the piezoelectric body, applying a DC bias to a resistive body mounted in the vicinity of a base portion of the cantilever to obtain a detection signal in accordance with deflection of the cantilever, and extracting the detecting signal through a differential amplifier. 
     Thus, in conventional configurations, a cantilever is externally excited using a piezoelectric body. This has resulted in a problem is that transmission of vibrations tends to become unstable depending on the state of mechanical coupling between a piezoelectric body and a cantilever and in that the provision of an additional piezoelectric body makes the configuration of an apparatus complicated. 
     Further, since deflection of a cantilever has been detected by applying a DC bias to a resistive body used as a distortion sensor and by detecting a resultant change in a voltage as the deflection of the cantilever, sensitivity has been poor, and accurate measurement has not been possible because of significant variations attributable to the mounting state. 
     An operating point is present in the vicinity of a resonance frequency of a cantilever, and it is therefore difficult to achieve reproducibility of the operating point because of variations of the characteristics of the cantilever, which has resulted in another problem in that a stable operation can not be expected. 
     It is an object of the invention to provide a self-exciting and self-detecting probe in which the above-described problems with the prior art can be solved and a scanning probe apparatus utilizing the same. 
     SUMMARY OF THE INVENTION 
     In order to solve the above-described problem, according to the invention, there is provided a scanning probe apparatus for measuring a sample by vibrating a lever of a probe, characterized in that a probe having a resistive body provided on the lever thereof is used and in that a periodic bias signal is applied to the resistive body to vibrate the lever. 
     The resistive body may be provided by doping the lever with an impurity by means of ion implantation. Variations in the transmission of vibrations can be eliminated to achieve more stable excitation compared to excitation using an externally attached piezoelectric body. The period of the bias signal can be arbitrarily chosen, which makes it easier to set an operating point different from the resonance point of the lever. 
     In the above-described configuration, the resistive body may be used as a detecting element for detecting distortion of the lever. 
     The above-described configuration may be modified by providing a separate detecting element for detecting distortion of the lever in addition to the resistive body. 
     In the above-described configuration, a reference resistive body having the same configuration as the resistive body may be provided in the vicinity of the lever, and the reference resistive body may be used for temperature compensation for detecting distortion of the lever. 
     When the reference resistive body is used for temperature compensation during the detection of distortion of the lever, a configuration may be employed in which the same AC bias is applied to both of the reference bodies. 
     If a bias signal having a periodically changing amplitude, such as an alternating current, is used to detect distortion of the lever, the detection can be carried out with a preferable signal-to-noise ratio, and highly accurate measurement can be performed according to various wave detecting methods using components such as amplitudes, phases, and frequencies. This makes it possible to detect signals at low levels and to provide a scanning probe apparatus capable of measurement with high resolution and accuracy. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an embodiment of a self-exciting and self-detecting probe according to the invention. 
     FIGS. 2A through 2D are graphs showing examples of operations of the probe shown in FIG.  1 . 
     FIG. 3 is an enlarged plan view of a major part of another embodiment of a self-exciting and self-detecting probe according to the invention. 
     FIG. 4 is a diagram showing an example of a circuit configuration for causing self-excitation of the probe shown in FIG.  3  and for detecting resultant displacement of a lever. 
     FIG. 5 is an enlarged plan vie w of a major part of a modification of the probe shown in FIG.  1 . 
     FIG. 6 is an enlarged plan view of a major part of a modification of the probe shown in FIG.  5 . 
     FIG. 7 is a circuit diagram of an example of a circuit configuration for detecting displacement of a lever in the probes shown in FIGS. 5 and 6. 
     FIG. 8 is a block diagram showing a configuration of a scanning probe apparatus. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     An embodiment of the present invention will now be described in detail with reference to the drawings. 
     FIG. 1 shows an embodiment of a self-exciting and self-detecting probe according to the invention. A probe  1  shown in FIG. 1 is configured as a probe to be used in a scanning probe microscope and can be manufactured, for example, using photolithography employing an SOI wafer or the like. The probe  1  is constituted by a lever  3  integrally extending from a support portion  2  formed by etching an SOI wafer, and a scanning needle  4  is provided at a free end of the lever  3  such that it is integral with the lever  3 . The scanning needle  4  may be a body which is separate from the lever  3 . The lever  3  may be configured without the scanning needle  4 . 
     A resistive body  5  is provided in the vicinity of a base portion of the lever  3 . In the present embodiment, the resistive body  5  is formed by a pair of resistive elements  5 A and  5 B provided such that they extend in the longitudinal direction of the lever  3 , and ends  5 A a  and  5 B a  of the respective resistive elements  5 A and  5 B are electrically connected by a metal wire  6 . As will be described later, the resistive elements  5 A and  5 B are provided for exciting the lever  3  and detecting distortion of the lever  3 . The resistive elements  5 A and  5 B are formed in a band-like configuration having a constant width by doping the lever  3  with an impurity (boron, phosphorus, or the like) by means of ion implantation or the like. 
     A pair of metal wires  7  and  8  in the form of thin films are deposited on the support portion  2  of the probe  1 , and ends  7 A and  8 A of the respective metal wires  7  and  8  are electrically connected to other ends  5 A b  and  5 B b  of the resistive elements  5 A and  5 B. Other ends  7 B and  8 B of the respective metal wires  7  and  8  are terminal portions to allow a configuration in which the resistive elements  5 A and  5 B can be electrically connected to an external circuit through the ends  7 B and  8 B. 
     In the present embodiment, any of the metal wires  6 ,  7 , and  8  are formed as a metal film using aluminum. 
     Since the probe  1  has the above-described configuration, the probe  1  may be used in a dynamic measurement mode in which it is set in a scanning probe microscope and in which measurement is carried out by vibrating the lever  3 . Specifically, when a voltage signal having a periodically changing level, e.g., an AC voltage current is applied between the ends  7 B and  8 B as an excitation voltage signal KV as an electrical energy for excitation, an AC current that flows through the resistive body  5  in response to the excitation voltage signal KV causes the lever  3  to displace (vibrate) at a frequency f 1  which is the same frequency as that of the excitation voltage signal KV. 
     FIGS. 2A and 2B show an example of an operation of the probe  1 . While the excitation voltage signal KV is a signal in the form of a sine wave whose level periodically changes between  0  and Va volts in the example shown in FIGS. 2A through 2D, a voltage signal in the form of a triangular wave, a rectangular wave, or the like may obviously be used instead. Regardless of the form of the excitation voltage signal KV, displacement of the lever  3  can be caused in accordance with periodic changes in the excitation voltage signal KV to allow self-excitation of the lever  3 . The amplitude of the vibration of the lever  3  can be adjusted by the voltage level of the excitation voltage signal KV. 
     Since the probe  1  is a self-exciting type in which the lever  3  is excited by applying a voltage signal for excitation to the resistive body  5  formed as described above, the excitation can be controlled with higher controllability than in conventional configurations for external excitation, which makes it possible to eliminate variation of the transmission of vibrations, thereby allowing excitation of the lever  3  with high accuracy and preferable reproducibility. As a result, signal detection can be performed with high sensitivity and accuracy. 
     Since the period of vibration of the lever  3  is determined by the frequency of the excitation voltage signal KV, it may be any frequency other than the resonance frequency of the lever  3 . However, this frequency is preferably a value in the vicinity of the resonance point of the lever  3  to allow the lever  3  to have a greater amplitude which provides improved detection sensitivity. 
     In the probe  1 , the resistive body  5  is also used for detecting distortion of the lever  3 . This is utilization of the fact that the resistance of the resistive body  5  change in accordance with distortion of the lever  3 , and a circuit for measuring the resistance of the resistive body  5  is connected between the ends  7 B and  8 B of the respective metal wires  7  and  8 . The circuit for measuring the resistance of the resistive body  5  may be similar to those used for conventional self-detecting probes. 
     The vibration frequency of the lever  3  is changed by a frequency offset of the excitation voltage signal KV. For example, an operation is possible in which the excitation voltage signal KV has a frequency f 2  as shown in FIG.  2 C and the vibration frequency of the lever  3  is 2×f 2  as shown in FIG.  2 D. However, an operation in which the frequency of the excitation voltage signal KV is the same as the vibration frequency of the lever  3  is preferred because the mode of operation shown in FIGS. 2A and 2B allows simpler detection of distortion based on a signal from the resistive body  5 . 
     FIG. 3 shows another embodiment of a self-exciting and self-detecting probe according to the invention. A probe  11  shown in FIG. 3 has the same basic configuration as that of the probe  1  shown in FIG. 1 except that an excitation resistive body for exciting a lever  3  and a displacement-detecting resistive body for detecting distortion (displacement) of the lever  3  are provided separately. Therefore, parts of the probe  11  having corresponding parts in the probe  1  are indicated by like reference numbers and will not be described here. 
     In FIG. 3,  12  represents an excitation resistive body formed by excitation resistive elements  12 A and  12 B;  13 ,  14 , and  15  represent metal wires for connection;  16  represents a detecting resistive body formed by detecting resistive elements  16 A and  16 B; and  17 ,  18 , and  19  represent metal wires for connection. They are fabricated similarly to the resistive body  5  and metal wires  6 ,  7 , and  8  shown in FIG. 1 using the same fabrication techniques. 
     Since the excitation resistive body  12  and the detecting resistive body  16  are provided as separate bodies in the probe  11  as described above, an excitation voltage signal KV can be externally applied to the metal wires  14  and  15  to excite the lever  3 , and resultant displacement of the lever  3  can be electrically detected by a detection circuit connected to the metal wires  18  and  19 . The detection circuit connected to the metal wires  18  and  19  may be any of well known circuits which are used in conventional self-detecting probes. 
     FIG. 4 shows an example of a configuration of a circuit for causing self-excitation of the probe  11  and for detecting resultant displacement of the;lever  3 . In FIG. 4,  21  represents a voltage generator for applying the excitation voltage signal KV for excitation to the excitation resistive elements  12 A and  12 B of the excitation resistive body  12 , and the reference number  22  generally represents a displacement detection circuit connected to the detecting resistive elements  16 A and  16 B of the detecting resistive body  16  for detecting displacement of the lever  3  from changes in the resistance of the detecting resistive elements  16 A and  16 B. 
     The displacement detection circuit  22  has resistors  23  and  24  which form a bridge circuit with the detecting resistive elements  16 A and  16 B. The resistor  23  is a variable resistor, and a DC voltage VB is supplied from a separate variable voltage source  27  for biasing between a variable contact of the same and the ground. The voltage across the resistor  23  is input to a differential amplifier  28 , and a voltage signal indicating displacement of the lever  3  is obtained at the output of the differential amplifier  28 . 
     FIG. 5 shows a modification of the probe  1  shown in FIG. 1. A probe  31  shown in FIG. 5 is different from the probe  1  in that a reference lever  32  is provided at a support portion 2  and in that a reference resistive body  33  is provided at the reference lever  32 . The reference resistive body  33  is configured similarly to the resistive body  5  and is formed by resistive elements  33 A and  33 B which are connected by metal wires  34 ,  35 ,  36 . 
     Since the probe  31  has the reference resistive body  33 , the use of the same makes it possible to detect displacement of the lever  3  accurately with the effect of thermal drifts of the reference resistive body  33  eliminated. The position of the reference lever  32  is not limited to the position shown in FIG.  5 . 
     FIG. 6 shows an example of a configuration in which the reference lever is provided in a support portion  2 . Referring to FIG. 6, in a probe  41 , a reference lever  42  is provided in a support portion  2 ; a reference resistive body  43  formed by resistive elements  43 A and  43 B is formed on the reference lever  42 ; and metal wires  44 ,  45 ,  46  are formed. The probes  31  and  41  are different in that the reference lever  42  is subjected to substantially no vibration while the reference lever  32  vibrates similarly to the lever  3  when the latter vibrates. 
     FIG. 7 shows an example of a circuit configuration for detecting displacement of the lever  3  in the probe  31  or  41  having the reference resistive body  33  or  34  as shown in FIGS. 5 and 6. 
     A detection circuit  51  has a variable resistor  52  connected between a resistive body  5  and a reference resistive body  33 , and one end of each of the resistive body  5  and reference resistive body  33  is grounded to configure a resistance bridge circuit formed by the resistive body  5 , reference resistive body  33 , and variable resistor  52 . An AC voltage AC for a bias signal is applied between a movable contact of the variable resistor  52  and the ground by an AC power source  53 . The voltage across the variable resistor  52  is input to a differential amplifier  54 , and a voltage signal indicating displacement of the lever  3  is obtained at the output of the differential amplifier  54 . 
     In the above-described circuit configuration, since the same resistance drifts as caused at the resistive body  5  by temperature changes occur at the reference resistive body  33 , those drifts are cancelled at the input of the differential amplifier  54 . Therefore, only voltage changes caused at the resistive body  5  by displacement of the lever  3  are obtained at the output of the differential amplifier  54 . 
     The same effect can be achieved by using the reference resistive body  43  instead of the reference resistive body  33 . However, the use of the reference resistive body  33  results in a great amount of cancellation of the drifts because the reference resistive body  33  is under the influence of vibrations of the reference lever  32 . On the contrary, when the reference resistive body  43  according to the configuration shown in FIG. 6 is used, the reference lever  42  is not subjected to vibrations similar to those at the lever  3 , the amount of drift cancellation is reduced accordingly. 
     While a circuit configuration for detecting displacement of the lever  3  using a resistive element has been described above, the amplitude, phase, and frequency of the lever  3  vary because the interaction between the sample and the scanning needle is affected by changes in the distance between them, the materials of them and the like. Therefore, a circuit configuration may be employed in which displacement of the lever  3  is detected based on the amplitude, phase, and frequency of a detection signal obtained from the resistive body. While a high signal-to-noise ratio is required for detection in such a manner, a circuit configuration as shown in FIG. 7 makes it possible to detect displacement of the lever  3  with high accuracy using the above-described various wave-detecting methods because it allows measurement with a high signal-to-noise ratio. 
     FIG. 8 is a block diagram showing a configuration of a scanning probe apparatus embodying the present invention. A sample  60  is placed on a three-dimensional sample stage  73 , and a scanning needle  4  of a probe  1  having a configuration according to the invention is provided above the sample  60  in a face-to-face relationship. A detecting portion  71  is formed by a bias circuit and an amplifier. It applies bias and excitation signals to the probe  1  and amplifies a signal generated as a result of distortion of the probe. A calculation portion  72  performs calculations to obtain a desired signal and converts changes in amplitude, phase, frequency, and the like into a displacement signal S 1 . A control portion  75  controls an actuator drive amplifier  70  such that the displacement signal S 1  approaches  0 . The output signal of the control portion  75  is supplied to a controller  77  as a luminance signal (Z-signal). A scan signal generating portion  76  supplies a jog signal for jogging the sample  60  in X- and Y-directions to the actuator drive amplifier  70  and supplies a raster scan signal to the controller  77 . The controller  77  may be easily configured using a personal computer or the like. It has the functions of controlling the apparatus as a whole with control signals and displaying X-, Y-, and Z-signals as a three-dimensional image. Further, arithmetic processes may be performed by dedicated software to convert the signals, thereby providing various kinds of information on a surface. 
     The present invention makes it possible to perform measurement with higher sensitivity and accuracy compared to conventional methods relying upon external excitation because variation of the transmission of vibrations is eliminated and excitation can be preferably controlled. With a configuration in which displacement is detected using a bias signal having a periodically changing vibration amplitude like an alternating current, displacement can be detected using amplitude, phase, and frequency components of a detection signal obtained by a resistive body, which allows measurement with high resolution and performance.