JP5061049B2 - Fine shape measuring device - Google Patents

Description

  The present invention relates to a fine shape measuring apparatus, and more particularly to improvement of an atomic force measuring means between a probe to be used and a surface to be measured.

  Conventionally, a shape measuring device such as a scanning probe microscope has been used to measure the fine shape of the surface to be measured, and an atomic force microscope is one of the scanning probe microscopes. The atomic force microscope detects the atomic force acting between the surface to be measured and the probe provided on the lower surface of the free end of the cantilever, and the fixed end of the cantilever is fixed so that the atomic force is constant. By scanning the surface to be measured with the probe while moving up and down, the uneven shape of the surface to be measured is measured. In many cases, a high-sensitivity optical lever method is used to detect the displacement of the probe due to the uneven shape of the surface to be measured (see, for example, Patent Document 1).

  FIG. 1 is a view showing a method for measuring the uneven shape of the surface S to be measured by an optical lever type atomic force microscope 10. That is, the tip of the probe 2 provided on the lower surface of the free end portion of the flexible cantilever 4 is in contact with the surface S to be measured, and the laser beam from the upper laser light source 6 is applied to the back side of the probe 2 in the cantilever 4. The light 7 is irradiated, and the reflected light is received by, for example, a four-divided photo detector 8. The atomic force is detected from the amount of bending of the cantilever 4.

  In measurement, the measurement surface S is scanned with the probe 2 while moving the measurement surface S in the X-axis direction. For example, the reflection angle of the laser beam 7 differs depending on the position of the probe 2 and the deflection angle of the cantilever 4 when the measured surface S is moved from the one-dot chain line position to the solid line position, and the laser beam in the photo detector 8 is different. Although the light receiving position of 7 is different, the probe 2 is scanned by moving the root of the cantilever 4 up and down in FIG. 1 so that the bending of the cantilever 4 is constant and measuring the amount of movement up and down with a displacement sensor. It is possible to measure the change in the height of the surface S to be measured accompanying the above.

  In addition to the above, the cantilever 4 is vibrated at a frequency in the vicinity of the resonance frequency, the vibrating probe 2 is scanned close to the measured surface S, and the atomic force acting between the probe 2 and the measured surface S is measured. The change in the vibration state of the probe 2 caused by the change can be detected by the optical lever method, and the cantilever 4 can be moved up and down so that the atomic force is constant, thereby measuring the unevenness of the measurement surface S. In any of the above cases, two displacement sensors, that is, a sensor for detecting the amount of vertical displacement caused by the vibration of the probe 2 and a sensor for detecting the amount of vertical displacement of the cantilever 4 are required.

  In contrast, the inventors of the present application have proposed a shape measuring device that can detect both the amount of vertical displacement of the probe and the amount of vertical displacement of the cantilever with a single displacement sensor (Patent Literature). 2). FIG. 2 is a diagram showing the configuration of the shape measuring apparatus 20. That is, the shape measuring device 20 includes a probe 12, a cantilever 14, a minute vibration generating means 16 that vibrates the probe 12, a cantilever position control means 17 that moves the cantilever 14 up and down, and a displacement sensor 21 of the probe 12. The measurement object 24 having the measurement surface S is placed on the XY stage 30.

  The micro-vibration generating means 16 has a cantilever 14 that is cantilevered. The cantilever 14 has a frequency higher than the sampling frequency of measurement data when measuring the uneven shape of the surface S to be measured, usually at a frequency near the resonance frequency of the cantilever 14. Is vibrated to vibrate the probe 12 in the vertical direction. The scanning means 18 moves the XY stage 30 horizontally in the X direction to cause the probe 12 to scan on the surface S to be measured, and then moves the XY stage 30 at a certain interval in the Y direction to perform similar scanning. repeat. That is, by scanning, the probe 12 provided at the free end of the cantilever 14 is displaced in the vertical direction according to the unevenness of the measurement surface S while being slightly vibrated in the vertical direction. Then, the cantilever position control means 17 raises and lowers the position of the minute vibration generating means 16 so that the atomic force between the measured surface S and the probe 12 becomes constant within an allowable range. Move the position up and down.

  The displacement sensor 21 is a displacement meter using a laser interferometer, and the laser light source 34 and the beam splitter 36 are installed independently of the XY stage 30. A reference surface (reference mirror) 38, which is another constituent member constituting the interference system, is provided on the XY stage 30 and moves together with the XY stage 30. The coherent laser beam 50 from the laser light source 34 is irradiated toward the back surface position of the probe 12 in the cantilever 14.

  The laser light 50 passes through the beam splitter 36 and reaches the reference surface (reference mirror) 38, but a part of the incident laser light 50 is reflected directly above the reference surface 38 to become reference light 52. The remaining laser light 50 passes through the reference surface 36 and the collimating lens 40, reaches the cantilever 14, and is reflected immediately above to become measurement light 54. The interference signal 56 between the reference beam 52 and the measuring beam 54 is reflected by the half mirror of the beam splitter 36 and sent to the probe displacement signal output circuit 42. The probe displacement signal output circuit 42 outputs the probe displacement signal 26. Input to the signal processing circuit 22C.

  The signal processing circuit 22 </ b> C obtains the amplitude of the probe vibration corresponding to the atomic force between the measurement surface S and the probe 12 from the high frequency component in the probe displacement signal 26. The signal processing circuit 22C feeds back the vibration amplitude to the cantilever position control means 17 so as to keep the probe vibration amplitude constant within an allowable range. The signal processing circuit 22 </ b> C acquires the uneven shape of the measurement surface S from the low frequency component in the probe displacement signal 26.

  A probe displacement signal 26 obtained by the shape measuring apparatus 20 is shown in FIG. The left diagram in FIG. 3A shows the uneven shape of the measurement surface S, and the right diagram in FIG. 3A shows the probe displacement signal 26 obtained when the measurement surface S is scanned by the probe 12. The scanning is performed by, for example, bringing the probe 12 close to the vicinity of the surface S to be measured while causing the cantilever 14 to slightly vibrate up and down by the fine vibration generating means 16 made of, for example, a piezoelectric element. ) Is kept constant, and the measurement surface S is scanned with the probe 12. That is, the probe displacement signal needle 26 indicates a minute displacement of the probe 12 and at the same time a displacement corresponding to the uneven shape of the measurement surface S.

  Since the cantilever 14 is vibrated at a frequency close to the resonance frequency of the cantilever 14 by the minute vibration generating means 16 that holds the fixed end thereof, the probe 12 is caused to vibrate up and down, as shown in FIG. 3B. The displacement signal needle 26 can be signal-processed and separated into a high-frequency component 66 based on minute vibrations of the probe 12 and a low-frequency component 68 based on the uneven shape of the measurement surface S.

  As shown in FIG. 3C, the amplitude La of the high frequency component 66 in the probe displacement signal 26 decreases as the probe 12 is brought closer to the surface S to be measured. Therefore, the atomic force can be kept constant by controlling the distance or contact force between the probe 12 and the measured surface S so that the amplitude La becomes a certain value La ′. FIG. 4 shows signal processing means in the signal processing circuit 22C for separating the probe displacement signal 26 into the high frequency component 66 and the low frequency component 68 shown in FIG. 3B.

  As shown in FIG. 4, the signal processing circuit 22C divides the probe displacement signal 26 output from the probe displacement signal output circuit 42 into two, one of which extracts a high frequency component 66 through a high pass filter 72, The amplitude of the probe vibration is acquired by the amplitude detection circuit 76 to which the component 66 is input. The amplitude of the probe vibration is fed back to the cantilever position control means 17 to raise and lower the position of the cantilever 14.

  The other divided portion passes through a low-pass filter 74, and the obtained low-frequency component 68 is input to the shape calculating means 80, and the uneven shape of the measured surface S is calculated.

JP 2002-181687 A Japanese Patent Application No. 2007-188070

  However, when the displacement of the probe is measured by the laser interferometer 21, the signal output from the laser interferometer 21 is the interference fringe intensity information, and normally, as shown in FIG. Are output as a two-phase sine wave having a phase difference of 90 degrees. This two-phase sine wave is inserted into an interpolation circuit to form a two-phase square wave. For example, the rising and falling edges of the two-phase square wave are counted by a counter, and can be converted into digital displacement information.

  Accordingly, the probe displacement signal 26 input to the signal processing circuit 22C from the probe displacement signal output circuit 42 shown in FIGS. 2 and 4 is usually a digital signal, and the probe displacement signal 26 is transmitted to the probe 12 as shown in FIG. Digital signal processing is required to separate the vibration component and the vertical movement component of the cantilever 14 due to the concavo-convex component of the surface S to be measured, but since the resonance frequency of the cantilever 14 is about several hundred kHz, Requires a high-speed digital signal processing technique, and there is a problem that development costs a lot.

  In addition to the above, it is conceivable that the displacement information output from the counter is converted to analog by a D / A converter, and the subsequent signal processing is performed by a general analog circuit. In that case, the measurement range of the shape measuring apparatus is limited by the resolution of the D / A converter. Normally, a counter having a resolution of 24 bits or higher is readily available, whereas the resolution of a high-speed sampling type D / A converter capable of supporting the shape measuring apparatus 20 of FIG. 2 is about 16 bits. . Therefore, this D / A conversion method has a problem that the measurement range becomes extremely narrow.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to detect both the displacement of the probe vibration and the vertical displacement of the cantilever based on the concavo-convex shape of the surface to be measured by a single displacement sensor. It is an object of the present invention to provide a shape measuring device that does not require advanced digital processing when separating displacement information of the probe into a vibration component of the probe and an uneven component of the surface to be measured. .

A shape measuring apparatus according to claim 1 for achieving the object of the present invention comprises a probe provided on the lower surface of the free end of the cantilever, and a fine vibration generating means for vibrating the probe up and down via the cantilever. A scanning means for scanning the probe on the surface to be measured via the cantilever, and a fixed end of the cantilever at a right angle to the scanning direction of the probe so that the atomic force acting between the probe and the surface to be measured is constant. A cantilever position control means that moves up and down in any direction, a laser interferometer that is a probe displacement sensor and generates a probe displacement signal, and a two-phase sine wave that is a probe displacement signal is branched into two, one of which is Probe displacement information is obtained by signal processing in a state that includes a high-frequency component corresponding to the vibration of the probe in the probe displacement signal and a low-frequency component corresponding to the uneven shape of the measured surface, and the other is a low-pass High frequency components were removed through a filter Signal processing means for acquiring the unevenness information of the measured surface in the probe displacement signal and extracting the vibration information of the probe from the difference between the displacement information of the probe and the unevenness information of the measured surface; Is a shape measuring device.

  Such a shape measuring apparatus obtains the amplitude information of the probe from the high frequency component obtained by subtracting the low frequency component corresponding to the uneven shape of the measured surface from the probe displacement information including the high frequency component and the low frequency component. Since the probe displacement signal is separated into a high-frequency component and a low-frequency component and the vibration amplitude of the probe is obtained from the high-frequency component as in the conventional shape measurement device, the height for separation is high. High-speed digital processing is not required, apparatus cost is suppressed, and measurement cost is reduced.

The shape measuring apparatus according to claim 2 is the measuring apparatus according to claim 1, wherein the vibration information of the probe is extracted from the difference between the displacement information of the probe and the unevenness information of the surface to be measured by the signal processing means. The shape measuring device is set to calculate only the lower bits necessary for obtaining the amplitude information of the probe vibration.
Such a shape measuring apparatus accurately calculates the amplitude of the probe without sacrificing the measurement range even if only the lower bits corresponding to the amplitude of the probe vibration are calculated when extracting the vibration component of the probe. Can be sought.

  According to the shape measuring apparatus of the present invention, in the shape measuring apparatus that detects the displacement of the probe vibration and the displacement of the cantilever that moves up and down based on the uneven shape of the surface to be measured by one displacement sensor, The high-frequency component of the probe vibration is extracted by subtracting the low-frequency component from the probe displacement information including the corresponding high-frequency component and the low-frequency wave component corresponding to the displacement of the cantilever. Therefore, an advanced digital signal processing circuit is required. In addition, the amplitude of the probe vibration can be measured without sacrificing the measurement range, and the measurement cost can be greatly reduced.

  The probe constituting the shape measuring apparatus of the present invention is provided on the lower surface of the free end of the cantilever, and while being vibrated up and down by the minute vibration generating means via the cantilever, the surface to be measured is also scanned by the scanning means via the cantilever. Scanned up. Note that the micro-vibration generating means is generally a piezoelectric body and vibrates in the vicinity of the cantilever resonance frequency. At the time of scanning, the cantilever has a fixed end portion in the vertical direction perpendicular to the scanning direction by the cantilever position control means so that the distance or contact force between the measurement surface having a concavo-convex shape and the probe is constant. Moved to.

  As described above, the probe is displaced by the vibration received and is displaced by the vertical movement of the cantilever. The displacement of the probe is observed by a Michelson interferometer laser interferometer as a displacement sensor. The laser light source and the beam splitter of the laser interferometer are installed independently of the XY stage on which the measurement target having the measurement surface is placed, the reference mirror is installed on the XY stage, and the laser beam Among them, an interference signal between the reference light reflected directly above the reference mirror and the measurement light transmitted through the reference mirror and the collimating lens and reflected directly above the cantilever is sent to the probe displacement signal output circuit. A probe displacement signal is input from the needle displacement signal output circuit to the signal processing circuit. These are the same as the shape measuring apparatus of Patent Document 2 shown in FIG. Therefore, FIG. 2 is used for the following description of the shape measuring apparatus of the present invention.

  The shape measuring apparatus of the present invention is different from the shape measuring apparatus of Patent Document 2 in the signal processing means in the signal processing circuit. That is, the signal processing means in the shape measuring apparatus of the present invention branches the probe displacement signal containing the high frequency component and the low frequency component into two as analog signals, and is one of the branched probe displacement signals. A first interpolation circuit to which a phase sine wave is input and a two-phase square wave output from the first interpolation circuit are counted, and displacement information of the probe composed of a high frequency component and a low frequency component is output. 1 counter, a low-pass filter that allows the other branched probe displacement signal to pass through, a second interpolation circuit that receives a probe displacement signal that has passed through the low-pass filter and has a high-frequency component cut, and a second internal circuit A second counter that counts the two-phase square wave output from the insertion circuit and outputs the unevenness information of the surface to be measured, the displacement information of the probe comprising the high frequency component and the low frequency component, and the low frequency measured Surface unevenness information is input, A subtracting circuit for outputting a vibration information of the needle, made of.

  The amplitude information of the probe vibration from the subtraction circuit is input to the amplitude detection circuit, the amplitude of the probe vibration output from the amplitude detection circuit is input to the cantilever position control means, and the amplitude that is input to the cantilever position control means By moving the cantilever up and down according to the change in the distance, the distance or contact force between the probe and the surface to be measured, that is, the atomic force is controlled to be constant.

  FIG. 6 shows a signal processing circuit for detecting the vibration amplitude of the probe 12 input to the cantilever position control means 17 in the shape measuring apparatus 80 of the present invention for measuring the uneven shape of the surface S to be measured based on the atomic force. It is a figure which shows the signal processing means in 22A, and replaces the signal processing means in the signal processing circuit 22C shown in FIG. With reference to FIG. 2, the interference signal 56 from the displacement sensor 21, which is a laser interferometer, is obtained from a high frequency component due to the vibration of the probe 12 from the probe displacement signal output circuit 42 and a low frequency component due to the unevenness of the surface S to be measured. To output a two-phase sine wave. In the shape measuring apparatus 80 of the present invention, as shown in FIG. 6, the two-phase sine wave is branched into two, and one of the branched waves is directly input to the first interpolation circuit 81, and the first interpolation circuit A two-phase square wave I is output from 81. The other branched part is input to the second interpolation circuit 86 after passing through the low-pass filter 85 in order to remove the high-frequency component due to the vibration of the probe 12, and the two-phase square wave II is input from the second interpolation circuit 86. Is output.

  The above-described two-phase square wave I is input to the first counter 83 and probe displacement information (displacement information I) 84 composed of a high-frequency component due to the vibration of the probe 12 and a low-frequency component due to the unevenness of the measured surface S is obtained. can get. In addition, the two-phase square wave II is input to the second counter 88, and unevenness information (displacement information II) 89 of the measurement surface S, which is a low frequency component, is obtained. Then, the vibration information 91 of the probe 12 is obtained by inputting the displacement information 84 of the probe and the unevenness information 89 of the measurement surface S to a subtracting circuit 90 combined with a general-purpose logic IC, for example. The vibration information 91 is converted into an analog signal by an amplitude detection circuit 92 (for example, a circuit that detects a change in vibration amplitude using a widely used full-wave rectifier circuit or the like), and probe vibrations. Can be obtained. Then, the vibration amplitude is fed back to the cantilever position control means 17 so that the surface to be measured S is scanned with the probe 12 while keeping the distance or contact force between the probe 12 and the surface to be measured S constant. it can.

  As described above, according to the shape measuring apparatus 80 of the present invention, the displacement information of the probe can be obtained from the high-frequency component due to the probe vibration and the unevenness of the measured surface S without constructing an advanced high-speed digital signal processing circuit. Can be easily separated into low frequency components.

  In such a shape measuring apparatus, in order to take a wide measurement range, a counter having a high resolution of, for example, about 24 bits or more is often used, but the amplitude of the probe 12 is large. Since the amplitude information 91 is obtained by taking the difference between the probe displacement information 84 and the unevenness information 89 of the surface S to be measured, all the outputs from the first counter 83 and the second counter 88 are It is not necessary to calculate the digit of, and the lower bit in a range that can correspond to the amplitude of the probe vibration may be appropriately selected and calculated. As a result, the number of digits in the subtraction circuit 90 can be reduced to simplify the calculation.

  The shape measuring apparatus of the present invention can be used for various scanning probe microscopes including an atomic force microscope and a scanning tunneling microscope.

It is a figure which shows the measuring method of the unevenness | corrugation of the to-be-measured surface by the optical lever system of an atomic force microscope. It is a figure which shows the structure of the conventional shape measuring apparatus which detects the displacement amount of a probe vibration and the displacement amount of the cantilever by the unevenness | corrugation of a to-be-measured surface by one displacement sensor. FIG. 3A shows displacement data in one displacement sensor of a conventional shape measuring apparatus, and FIG. 3B shows a state in which the displacement data is separated into a high-frequency component due to probe vibration and a low-frequency component due to irregularities on the surface to be measured. FIG. 3C shows a case where the distance between the probe and the surface to be measured or the contact force is controlled to set the amplitude of the probe vibration to a constant value. It is a figure which shows the structure of the signal circuit isolate | separated into the high frequency component by said probe vibration, and the low frequency component by the unevenness | corrugation of a to-be-measured surface. A general method for obtaining displacement information by digitizing and counting analog signals from an optical interferometer as a displacement sensor will be described. It is a figure which shows the signal processing means which detects the amplitude of a probe vibration in the shape measuring apparatus of this invention.

Explanation of symbols

12 ... probe, 14 ... cantilever,
16 ... micro vibration generating means, 17 ... cantilever position control means,
18 ... scanning means, 20 ... conventional shape measuring device,
21 ... Displacement sensor (laser interferometer), 22A... Signal processing circuit of the device of the present invention,
22C ··· signal processing circuit of conventional device, 24 · · · measurement object,
26 ... probe displacement signal, 30 ... XY stage,
34 ... laser light source, 36 ... beam splitter,
38 ... reference surface (reference mirror), 40 ... collimating lens,
42 ... probe displacement signal output circuit, 50 ... laser beam,
52 ... Reference light, 54 ... Measurement light,
56 ... interference signal, 66 ... high frequency component,
68 ... low frequency component, 80 ... shape measuring apparatus of the present invention,
81: first interpolation circuit, 83: first counter,
84 ... displacement information of the probe, 85 ... low-pass filter,
86 ... second interpolation circuit, 88 ... second counter,
89... Unevenness information of measured surface, 90.
91: Probe vibration information, 92: Amplitude detection circuit,
S: surface to be measured,

Claims (2)

A probe provided on the lower surface of the free end of the cantilever;
Fine vibration generating means for vibrating the probe up and down via the cantilever;
Scanning means for scanning the probe on the surface to be measured via the cantilever;
A cantilever position control means for moving the fixed end of the cantilever up and down in a direction perpendicular to the scanning direction of the probe so as to make the atomic force acting between the probe and the surface to be measured constant,
A laser interferometer that is a displacement sensor of the probe and generates a probe displacement signal;
The two-phase sine wave which is the probe displacement signal is branched into two, one of which is a high frequency component corresponding to the vibration of the probe in the probe displacement signal and a low frequency corresponding to the uneven shape of the surface to be measured. The probe displacement information is obtained by performing signal processing in a state including the components, and the other is subjected to signal processing after removing the high-frequency component through a low-pass filter to perform unevenness information on the surface to be measured in the probe displacement signal. And a signal processing means for extracting the vibration information of the probe from the difference between the displacement information of the probe and the unevenness information of the surface to be measured. In the shape measuring apparatus according to claim 1,
Lower-order bits required to acquire amplitude information of the probe vibration when extracting the vibration information of the probe from the difference between the displacement information of the probe and the unevenness information of the measured surface by the signal processing means A shape measuring apparatus, which is set to calculate only.

Images