How to determine optical lever sensitivity

Abstract

To accurately determine the optical lever sensitivity on the spot while pressing a cantilever against an object under measurement.SOLUTION: An optical lever sensitivity computation method comprises: a measurement step of simultaneously measuring a contact resonance frequency, force applied to a cantilever, and a deflection amount of the object under measurement while the cantilever (2) is in contact with the object under measurement (11); and a computation step of computing the optical lever sensitivity based on measured values.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for determining optical lever sensitivity. [Background technology]

[0002] The optical lever method is known as a method for measuring the unevenness and displacement of an object. In the optical lever method, a needle is placed at the tip of a cantilever. The needle is brought into contact with the object to be measured, and the unevenness and displacement of the object are measured as the displacement of the cantilever. The displacement of the cantilever is measured by a photodiode that detects changes in the laser light that is irradiated onto the back of the cantilever and reflected. The photodiode used is, for example, a two- or four-segment type.

[0003] Patent Document 1 describes a scanning probe microscope that has a cantilever with a probe at its tip detachably attached to a mounting part, and is equipped with a measurement device that obtains information about the sample surface by bringing the probe close to the sample, and is configured to change the cantilever according to measurement conditions. The scanning probe microscope is equipped with a detection means that detects the type or manufacturing variation of the cantilever of the changed cantilever, a parameter determination means that determines parameter values ​​to be set in the measurement device based on the detection information obtained by the detection means, and a parameter change means that changes the parameter settings of the measurement device using the determined parameter values. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-308406 Summary of the Invention [Problem to be solved by the invention]

[0005] Generally, the displacement of the cantilever is proportional to the amount of light detected by the photodiode. The proportionality constant in this case is called the optical lever sensitivity.

[0006] The optical lever sensitivity is denoted as α. The optical lever sensitivity α can be calculated from the cantilever displacement and the proportionality coefficient of the photodiode for a sufficiently hard object to be measured, such as a material with zero strain from the free surface of the object to be measured. However, it is not known whether the object to be measured is actually sufficiently hard. Furthermore, if the object to be measured changes, the path of the light used for measurement will shift, which may change the optical lever sensitivity α. Furthermore, since the cantilever probe is pressed against the hard surface of the object to be measured, the shape of the tip may change, which may change the optical lever sensitivity α. Therefore, there is uncertainty when calculating the optical lever sensitivity α using this method.

[0007] The optical lever sensitivity α can also be found by fitting the natural vibration spectrum of the cantilever due to thermal excitation. However, in this case, the optical lever sensitivity α is calculated when the cantilever is not in contact with the object being measured. Therefore, when the cantilever is actually brought closer to the object being measured, the optical lever sensitivity α may change because the path of light changes. Furthermore, this method finds the optical lever sensitivity α by fitting based on a theoretical formula that assumes the shape of the probe, etc., so the results may change if the shape of the probe changes, and the error may be large.

[0008] The present invention has been made in view of the above circumstances, and provides a method for determining optical lever sensitivity, which determines the optical lever sensitivity on the spot with high accuracy while the cantilever is pressed against an object to be measured. [Means for solving the problem]

[0009] In order to achieve the above-mentioned object, the optical lever sensitivity calculation method according to the present invention is characterized by the following [1] to [4]. [1] a measuring step of simultaneously measuring the contact resonance frequency, the force applied to the cantilever, and the deflection of the object (11) while the cantilever (2) is in contact with the object (11); a calculation step of calculating the optical lever sensitivity based on the measured value; The optical lever sensitivity calculation method has the following. [2] In the calculation step, calculating a first calculated value based on the contact resonant frequency; calculating a second calculated value based on the force applied to the cantilever and the deflection of the object to be measured; calculating the optical lever sensitivity by comparing the first calculated value with the second calculated value; [1] The optical lever sensitivity calculation method described in [1]. [3] In the calculation step, The first calculated value,

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[0010] According to the configuration of [1] above, by simultaneously measuring the contact resonance frequency, the force applied to the cantilever, and the deflection of the object under test while the cantilever is pressed against the object under test, the optical lever sensitivity can be determined with high accuracy on the spot, for example, when setting up the object under test. According to the configuration [2] above, the optical lever sensitivity is determined based on a comparison between the two calculated values, so that the optical lever sensitivity can be determined with high accuracy. According to the configuration of [3] above, the elastic constant k of the object to be measured can be calculated using two formulas. s Since the optical lever sensitivity is determined by comparing the above, the optical lever sensitivity can be determined immediately on the spot using a general computer. According to the configuration [4] above, the optical lever sensitivity can be determined using a general spectrum analyzer that is used to analyze the frequency components of the detection signal of the photodiode. [Effects of the Invention]

[0011] According to the present invention, the optical lever sensitivity can be determined in situ with high accuracy while the cantilever is pressed against the object to be measured. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a conceptual diagram illustrating an example of the configuration of a measurement device according to an embodiment of the present disclosure. [Figure 2] FIG. 10 is a conceptual diagram for explaining various values ​​related to calculation of optical lever sensitivity according to an embodiment of the present disclosure. [Figure 3] 10 is a graph showing a measurement example according to an embodiment of the present disclosure. [Figure 4] 10 is a graph showing a measurement example according to an embodiment of the present disclosure. [Figure 5] 10 is a graph showing a measurement example according to an embodiment of the present disclosure. [Figure 6] 1 is a graph showing the degree of agreement between values ​​calculated using two measurement methods according to an embodiment of the present disclosure. [Figure 7] 1 is a graph showing the degree of agreement between values ​​calculated using two measurement methods according to an embodiment of the present disclosure. [Figure 8] 1 is a graph showing the degree of agreement between values ​​calculated using two measurement methods according to an embodiment of the present disclosure. [Figure 9] 10 is a flowchart showing a process for calculating an optical lever sensitivity α according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0014] 1 is a conceptual diagram showing an example configuration of a measurement device according to an embodiment of the present disclosure. The measurement device 100 is, for example, an atomic force microscope (AFM). The measurement device 100 may be a device other than an atomic force microscope.

[0015] As an example, the measurement device 100 includes a light source 1, a cantilever 2, a mirror 3, a photodiode 4, a calculator 5, a spectrum analyzer 6, a feedback circuit 8, and a stage 10. An object to be measured 11 is placed on the stage 10.

[0016] Light source 1 irradiates laser light onto the back surface of cantilever 2. The laser light reflected by the back surface of cantilever 2 is reflected again by mirror 3 and enters photodiode 4. A voltage value corresponding to the intensity of the light entering photodiode 4 is input to calculator 5.

[0017] A spectrum analyzer 6 acquires the voltage value output from the photodiode 4 and performs spectrum analysis. The spectrum analyzer 6 itself is a general-purpose analyzer, so a detailed description thereof will be omitted.

[0018] The control value output from the computer 5 is input to a feedback circuit 8 .

[0019] A Z control signal is sent from the feedback circuit 8 to the stage 10. Based on the Z control signal, the stage 10 is displaced in the Z-axis direction of the Cartesian coordinate system shown in Figure 1. Each time the stage 10 is moved slightly in the Z-axis direction, a spectrum is acquired by the spectrum analyzer 6 to determine the contact resonance frequency. A general spectrum analyzer used to analyze the frequency components of the detection signal from a photodiode can be used as the spectrum analyzer 6. In this case, the resolution bandwidth (RBW) is increased to speed up measurement at each position in the Z-axis direction.

[0020] After calculating the optical lever sensitivity α using the method described below, the calculator 5 sends an XY scanning signal to the stage 10. In response to this XY scanning signal, the stage 10 moves in a direction included in the plane formed by the X-axis and Y-axis directions. In other words, in response to the XY scanning signal, the stage 10 moves in a direction perpendicular to the Z-axis.

[0021] In this embodiment, the cantilever 2 is brought into contact with the object 11 to be measured and moved in the Z-axis direction, i.e., up and down, while simultaneously measuring the contact resonance frequency, the force applied to the cantilever, and the amount of deflection of the object to be measured.

[0022] As described above, the measuring device 100 according to this embodiment is configured to irradiate the cantilever 2 with a laser and detect the amount of reflected light with the photodiode 4. Here, the amount detected by the photodiode 4 is represented as DV.

[0023] When the cantilever 2 is pressed against the object 11, both the cantilever 2 and the object 11 bend. If the amount of deflection of the cantilever 2 is d, the parameter for calculating the amount of deflection d from the amount DV detected by the photodiode 4 is the optical lever sensitivity α. That is, the following equation holds:

[0024] d=α·DV

[0025] The computer 5 includes a control unit and a storage unit. The control unit is configured using, for example, a CPU (Central Processing Unit), an MPU (Micro Processing Unit), a DSP (Digital Signal Processor), or an FPGA (Field Programmable Gate Array).

[0026] The storage unit included in the computer 5 stores the programs executed by the control unit and various data used during execution. The storage unit may include a HDD, ROM, RAM, etc., and stores various programs (OS, application software, etc.) and various data executed by the control unit.

[0027] In this embodiment, an example will be described in which the calculation process of the optical lever sensitivity α is executed by a control unit included in the calculator 5. However, the calculation process of the optical lever sensitivity α may also be executed by a device other than the calculator 5, for example, a control unit included in the spectrum analyzer 6. Any device that executes the calculation process may obtain parameters necessary for the calculation and then calculate the optical lever sensitivity α using those parameters.

[0028] In this embodiment, the optical lever sensitivity α is calculated while measuring the physical properties of the DUT 11 with the spectrum analyzer 6. This calculation is performed, for example, first after the DUT 11 is set in the measurement device 100. However, the calculation may be performed at any other timing.

[0029] FIG. 2 is a conceptual diagram for explaining various values ​​related to the calculation of the optical lever sensitivity according to an embodiment of the present disclosure.

[0030] Various values ​​related to the calculation of the optical lever sensitivity are represented by the following symbols. δ: Distortion from the free surface of the object being measured, i.e., the amount of deflection of the object being measured k s : Elastic constant of the object to be measured d: cantilever deflection k c : cantilever spring constant F: Force acting on the object to be measured and the cantilever Z: Height change of the support point on the cantilever DV: Amount detected by the photodiode f0: natural frequency of the cantilever f CR : Contact resonance frequency of the cantilever when it is in contact with the object to be measured α: Optical lever sensitivity

[0031] The spring constant of the cantilever is k c Since is determined for each cantilever, it is known when calculating the optical lever sensitivity α. The natural frequency f0 of the cantilever is also determined for each cantilever, so it is known when calculating the optical lever sensitivity α. If the cantilever is not in contact with the object to be measured, the contact resonance frequency f CR is equal to the natural frequency f0. Also, the elastic constant k of the object being measured s is a function of δ.

[0032] 2, when the object to be measured moves in the Z-axis direction as shown in FIG. 1, Z may be defined as the displacement of the stage 10.

[0033] Let a0 be the height from the tip of the cantilever to the fulcrum when the cantilever is not distorted. Let a1 be the height from the tip of the cantilever to the fulcrum when the cantilever is distorted by a force F. In this case, the following equation holds:

[0034] d=a0-a1=α·DV δ=Zd=Z-α·DV F=k c ·d

[0035] (k s (First measurement method) In this embodiment, when measuring the deflection amount d of the cantilever 2, the laser light reflected from the cantilever 2 is detected by the photodiode 4. Then, the detection signal detected by the photodiode 4 is input to the spectrum analyzer 6, and the contact resonance frequency f CR Measure.

[0036] Elastic constant k of the object to be measured s is the contact resonance frequency f when the cantilever is in contact with the object to be measured. CR Once we have found this, we can determine the spring constant k of the cantilever. c and the natural frequency f0 can be calculated based on the following relational expression.

[0037]

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[0038] (k s (Second measurement method) On the other hand, the elastic constant k of the object to be measured s is calculated from the force acting on the cantilever and the deflection of the object being measured as follows:

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[0039] The calculator 5 calculates the elastic constant k of the object to be measured by the first and second measurement methods. s Calculator 5 calculates the two elastic constants k measured by the two measurement methods. s By comparing these, the optical lever sensitivity α is calculated with high accuracy.

[0040] 3 is a graph showing a measurement example according to an embodiment of the present disclosure. The horizontal axis in each graph represents time in seconds. The vertical axis in each graph represents the frequency f, the amount DV detected by the photodiode, and the change in height Z of the fulcrum of the cantilever. In the graphs where the vertical axis represents the frequency, the darker shaded areas represent the contact resonance frequency f. CRThe contact resonance frequency f CR On the other hand, there is a jump in about the contact resonance frequency f CR There is a jump out about.

[0041] 4 is a graph showing a measurement example according to an embodiment of the present disclosure. The horizontal axis of the graph shown in FIG. 4 represents time in seconds. FIG. 4 shows the DV and f around 0.00 seconds, which is the timing of the jump-in shown in FIG. 3. CR The values ​​shown are:

[0042] 5 is a graph showing a measurement example according to an embodiment of the present disclosure. The horizontal axis of the graph shown in FIG. 5 represents time in seconds. FIG. 5 shows the DV and f at around 0.59 seconds, which is the timing of the jump-out shown in FIG. 3. CR The values ​​shown are:

[0043] Figure 6 is a graph showing the degree of agreement between values ​​calculated using two measurement methods according to an embodiment of the present disclosure. Figure 7 is a graph showing the degree of agreement between values ​​calculated using two measurement methods according to an embodiment of the present disclosure. Figure 8 is a graph showing the degree of agreement between values ​​calculated using two measurement methods according to an embodiment of the present disclosure.

[0044] In Figures 6, 7, and 8, the squares with a filled center indicate first calculated values ​​based on the first measurement method, and the squares with an open center indicate second calculated values ​​based on the second measurement method.

[0045] In this example, the first and second calculated values ​​are k s k c The value divided by k c The values ​​before division by may be used as the first calculated value and the second calculated value. Values ​​other than these may also be used as the first calculated value and the second calculated value.

[0046] Figure 6 is a graph when the provisional value of α is determined to be 10.7, Figure 7 is a graph when the provisional value of α is determined to be 9.5, and Figure 8 is a graph when the provisional value of α is determined to be 8.3.

[0047] As is clear from Figures 6, 7, and 8, when the provisional value of α is 10.7 or 8.3, there is a discrepancy between the first calculated value based on the first measurement method and the second calculated value based on the second measurement method. On the other hand, when the provisional value of α is 9.5, there is little discrepancy between the first calculated value based on the first measurement method and the second calculated value based on the second measurement method. Therefore, Calculator 5 determines the value of α as 9.5, which has the smallest discrepancy. Note that α = 10.7 is equal to the value measured on the surface of a test object that is considered sufficiently hard. This indicates that the conventional calculation method is inaccurate.

[0048] FIG. 9 is a flowchart illustrating a calculation process of the optical lever sensitivity α according to an embodiment of the present disclosure.

[0049] Calculator 5 provisionally sets a provisional value for α (St101).

[0050] Calculator 5 calculates the value shown in the following formula (St102).

[0051]

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[0052] The above formula means that the absolute value of the difference between the first calculated value based on the first measurement method and the second calculated value based on the second measurement method is taken and then summed over the entire measurement in Figures 6, 7, and 8. In other words, the smaller the difference between the first calculated value and the second calculated value, the smaller the value of B.

[0053] Calculator 5 determines whether the calculated value of B is smaller than the stored value of B (St103). If the calculated value of B is smaller than the stored value of B (St103: YES), the process proceeds to step St104. If the calculated value of B is not smaller than the stored value of B (St103: NO), the process proceeds to step St105.

[0054] In step St104, the computer 5 stores the provisional value of α at this time. The storage destination may be a storage unit included in the computer 5, or an external storage medium accessible from the computer 5.

[0055] In step St105, the calculator 5 increases the provisional value of α by a predetermined amount.

[0056] Calculator 5 determines whether the provisional value of α exceeds a predetermined maximum value (St106). If the provisional value of α exceeds the maximum value (St106: YES), the process proceeds to step St107. If the value of α does not exceed the maximum value (St106: NO), the process returns to step St102, and the processes from step St102 onward are repeated based on the new provisional value of α after being increased in step St105.

[0057] In step St107, the calculator 5 determines the provisional value of α stored at this point as the α to be calculated.

[0058] The above flow corresponds to a process of initially setting the provisional value of α to a small value, and gradually increasing the provisional value of α to find the value of α that minimizes the deviation between the first calculated value and the second calculated value. However, it is also possible to initially set the provisional value of α to a large value, and then gradually decrease the provisional value of α in step St105 to find the value of α that minimizes the deviation between the first calculated value and the second calculated value.

[0059] 9 is an example of a process for calculating the optical lever sensitivity α by comparing the first calculated value with the second calculated value. The calculator 5 may calculate the optical lever sensitivity α based on an algorithm other than this.

[0060] By performing the above-described processing, the optical lever sensitivity calculation method according to this embodiment can accurately determine the optical lever sensitivity on the spot while the cantilever is pressed against the object to be measured.

[0061] Although various embodiments have been described above with reference to the drawings, it goes without saying that the present disclosure is not limited to such examples. It is clear to those skilled in the art that various modifications and alterations can be made within the scope of the claims, and it is understood that these also fall within the technical scope of the present disclosure. For example, the steps in the methods disclosed herein may be performed in any order as long as no contradictions arise. Furthermore, the components in the above embodiments may be combined in any order as long as they do not deviate from the spirit of the disclosure. [Explanation of symbols]

[0062] 1 light source 2 Cantilevers 3. Mirror 4 photodiodes 5 Calculator 6. Spectrum Analyzer 8 Feedback Circuit 10 stages 11 Object to be measured 100 Measuring Device

Claims

1. a measuring step of simultaneously measuring the contact resonance frequency, the force applied to the cantilever, and the deflection of the object (11) while the cantilever (2) is in contact with the object (11); a calculation step of calculating the optical lever sensitivity based on the measured value; The optical lever sensitivity calculation method has the following.

2. In the calculation step, calculating a first calculated value based on the contact resonant frequency; calculating a second calculated value based on the force applied to the cantilever and the deflection of the object to be measured; calculating the optical lever sensitivity by comparing the first calculated value with the second calculated value; The optical lever sensitivity calculation method according to claim 1 .

3. In the calculation step, The first calculated value, [Equation 1] Calculated as a value based on The second calculated value, [Equation 2] Calculated as a value based on k ｓ is the elastic constant of the object to be measured, k ｃ is the spring constant of the cantilever, f ＣＲ is the contact resonance frequency of the cantilever when it is in contact with the object to be measured, f ０ is the natural frequency of the cantilever, F is the force acting on the object to be measured and the cantilever, and δ is the amount of deflection of the object to be measured. The optical lever sensitivity calculation method according to claim 2 .

4. In the measuring step, the contact resonance frequency is measured by inputting a detection signal obtained by detecting laser light reflected from the cantilever by a photodiode (4) into a spectrum analyzer (6) when measuring the deflection of the cantilever. The optical lever sensitivity calculation method according to claim 1 .

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