Optical lever sensitivity determination method

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 and reflected from the back of the cantilever. For example, a photodiode divided into two or four segments is used.

[0003] Patent Document 1 describes a scanning probe microscope that has a cantilever with a probe at its tip detachably attached to an attachment section, is equipped with a measuring device that obtains information about the sample surface by bringing the probe close to a 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 the value of a parameter that is set in the measuring device based on the detection information obtained by the detection means, and a parameter change means that changes the setting value of the parameter of the measuring device with the determined parameter value. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2005-308406 A Summary of the Invention [Problem to be solved by the invention]

[0005] In general, 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 is sometimes calculated from the displacement of the cantilever and the proportionality coefficient of the photodiode for a sufficiently hard object to be measured, for example a material with zero distortion from the free surface of the object to be measured. However, it is not known whether the object to be measured is really hard enough. In addition, if the object to be measured changes, the path of the light for measurement shifts, which may change the optical lever sensitivity α. In addition, since the probe of the cantilever is pressed against the hard surface of the object to be measured, the shape of the needle tip may change, which may change the optical lever sensitivity α. Therefore, there is some uncertainty in 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. In this case, however, the optical lever sensitivity α is calculated when the cantilever is not in contact with the object to be measured. Therefore, when the cantilever is actually brought closer to the object to be measured, the optical lever sensitivity α may change because the path of the 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 if the shape of the probe changes, the results may change and the error may be large.

[0008] The present invention has been made in consideration 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, an 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; A calculation step of calculating the optical lever sensitivity based on the measured value; The optical lever sensitivity calculation method has the following features. [2] In the calculation step, A first calculation value is calculated 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 described in [1]. [3] In the calculation step, The first calculated value,

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

[0011] According to the present invention, the optical lever sensitivity can be determined on the spot with high accuracy while the cantilever is pressed against the object to be measured. [Brief description 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. [Diagram 2] 1 is a conceptual diagram for explaining various values ​​related to calculation of optical lever sensitivity according to an embodiment of the present disclosure. FIG. [Diagram 3] 11 is a graph showing a measurement example according to an embodiment of the present disclosure. [Figure 4] 11 is a graph showing a measurement example according to an embodiment of the present disclosure. [Diagram 5] 11 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] 11 is a flowchart showing a calculation process of an optical lever sensitivity α according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

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

[0015] As an example, the measurement apparatus 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] A light source 1 irradiates a laser beam toward the back surface of a cantilever 2. The laser beam reflected by the back surface of the cantilever 2 is reflected again by a mirror 3 and enters a photodiode 4. A voltage value according to the intensity of the light entering the photodiode 4 is input to a calculator 5.

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

[0018] The control value output from the calculator 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 FIG. 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 a 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 the measurement at each position in the Z-axis direction.

[0020] After the optical lever sensitivity α is calculated by the method described below, the calculator 5 transmits an XY scanning signal to the stage 10. In response to this XY scanning signal, the stage 10 moves in a direction included in a plane formed by the X-axis direction and the Y-axis direction. That is, 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 to be measured 11 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 apply a laser to the cantilever 2 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 bending of the cantilever 2 is d, the parameter for calculating the amount of bending d from the amount of bending DV detected by the photodiode 4 is the optical lever sensitivity α. In other words, 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 central processing unit (CPU), a micro processing unit (MPU), a digital signal processor (DSP), or a field programmable gate array (FPGA).

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

[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 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 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: deflection of the cantilever k c : Spring constant of the cantilever 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 s is a function of δ.

[0032] In addition, when the object to be measured moves in the Z-axis direction as shown in FIG. 1, instead of the cantilever moving in the Z-axis direction as shown in FIG.

[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 the application of force F. In this case, the following formula 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 a spectrum analyzer 6, whereby 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 the cantilever spring constant k c and the natural frequency f0 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 applied to the cantilever and the deflection of the object to be measured as follows:

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[0039] The calculator 5 calculates the elastic constant k of the object to be measured by the above-mentioned first measurement method and the second measurement method. s Calculator 5 calculates the two elastic constants k s By comparing these values, 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 vibration 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 vibration frequency, the darker 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 is time in seconds. FIG. 4 shows the DV and f 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 is 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] Fig. 6 is a graph showing the degree of agreement between values ​​calculated using two measurement methods according to an embodiment of the present disclosure. Fig. 7 is a graph showing the degree of agreement between values ​​calculated using two measurement methods according to an embodiment of the present disclosure. Fig. 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 may be used as the first and second calculated values. Values ​​other than these may be used as the first and second calculated values.

[0046] Fig. 6 is a graph in which the provisional value of α is determined to be 10.7, Fig. 7 is a graph in which the provisional value of α is determined to be 9.5, and Fig. 8 is a graph in which 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, the calculator 5 determines the value of α to be 9.5, which has the smallest discrepancy. Note that α=10.7 is equal to the value measured on the surface of the object to be measured that is considered to be sufficiently hard. In other words, this shows that the conventional calculation method is not accurate.

[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 the absolute value is added up for the entire measurement in Figures 6, 7, and 8. In other words, the smaller the deviation between the first calculated value and the second calculated value, the smaller the value of B.

[0053] The calculator 5 judges 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 point in 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 to be the α to be obtained.

[0058] The above flow corresponds to a process of initially setting the provisional value of α to a small value, gradually increasing the provisional value of α, and searching for α that minimizes the deviation between the first and second calculated values. 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 search for α that minimizes the deviation between the first and second calculated values.

[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 the above.

[0060] By carrying out the above-described processing, the optical lever sensitivity calculation method according to this embodiment can accurately determine the optical lever sensitivity on the spot with the cantilever 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 that a person skilled in the art can come up with various modified or amended examples within the scope of the claims, and it is understood that these also naturally belong to the technical scope of the present disclosure. For example, the steps in the method of the present disclosure can be performed in any order as long as there is no contradiction. In addition, the components in the above embodiments may be combined in any order as long as it does not deviate from the spirit of the disclosure. [Explanation of symbols]

[0062] 1 light source 2 Cantilever 3. Mirror 4 Photodiodes 5 Calculator 6. Spectrum Analyzer 8 Feedback Circuit 10 Stages 11 Object to be measured 100 Measuring Equipment

Claims

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

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, [0010] Calculated as a value based on The second calculated value, [0025] 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 the object to be measured is in contact with it, and 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 deflection of the object to be measured. The optical lever sensitivity calculation method according to claim 2 .

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

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