CN101393008A - PVDF-based tap-type high-sensitivity SPM probe and measurement method - Google Patents

Abstract

Tapping high-sensitivity SPM probe and measurement method based on PVDF, characterized in that the PVDF piezoelectric film is used as the vibrating beam, and the tungsten probe is used as the scanning probe; the vibrating beam is set as a simply supported beam with a curved radian The left and right ends of the sensor are respectively fixed to the front ends of the opposite piezoelectric actuators on both sides through the clamping structure, and the rear ends of the piezoelectric actuators on both sides are respectively fixed to the respective cantilever beams. The cantilever beams are suspended on the probe at one end on the frame; the scanning probe is fixedly set at the center of the lower surface of the vibrating beam. The invention uses the change of the electric charge output by the vibrating beam to represent the amplitude change of the vibrating beam caused by the micro-measurement force, and performs multi-point micro-measurement force tap scanning on the measured surface to realize the measurement of the sample surface. The invention can realize high spatial resolution and low measuring force, is suitable for various materials, and can be measured in air environment.

Description

PVDF-based tap-type high-sensitivity SPM probe and measurement method

technical field

The invention relates to a nanometer measurement device, more specifically a measuring head that can be used to form a scanning probe microscope system, so that the system can realize the feedback control of the nanometer resolution in the vertical direction in the measurement of the nanometer microscopic shape; it can form a micro-nano three-dimensional The nano-positioning probe of the coordinate measuring machine realizes the nano-positioning of the probe of the three-coordinate measuring machine; it can be used to form a scanning probe microscope system to realize the surface characteristics of soft and fragile samples (such as the surface of biological materials such as protein molecules) and steep steps Samples (such as semiconductor silicon micro-devices, micro-electro-mechanical devices, etc.) for non-destructive micro-surface topography measurement at nanoscale resolution; Measurement of micromechanical parts.

Background technique

In the past ten years, with the high-precision requirements of micro-semiconductor devices, MEMS, and nano-devices for the measurement of surface micro-topography, as well as the non-destructive requirements for the surface measurement of biological materials such as DNA and protein molecules, it is required that measuring instruments not only have nano-scale resolution, but also with the smallest possible measuring force.

The commonly used stylus profiler is a simple and reliable precision measuring instrument widely used in mechanical surface measurement. Its measurement range can reach tens of millimeters, but its vertical resolution can only reach tens of nanometers, and the measurement process The middle stylus is in continuous contact with the surface to be measured, and the lateral force is large, which is easy to cause scratches on the surface. It is not suitable for the measurement of soft materials and surfaces with steep microstructures. Although optical measurement systems such as confocal microscopes have the advantage of non-contact measurement, and their highest vertical resolution is close to 10 nanometers, their lateral resolution cannot be improved due to the limitation of the focus spot diameter, and they are not suitable for non-reflective materials Measurement. Although the scanning tunneling microscope (STM) has the advantages of sub-nanometer vertical resolution and non-contact measurement, it cannot be directly applied to insulating materials and materials whose surface is easily oxidized because the measurement current is greatly affected by the conductivity of the measured material. There are also high requirements on the vacuum degree of the measurement environment, so its application range is greatly limited. Although the atomic force microscope (AFM) is suitable for the measurement of various materials and parameters, and has sub-nanometer vertical resolution and nN-level measurement force, most of the probes used are silicon micro-cantilever type, and its effective length Short, tip curvature radius is large, and the cone angle is usually about 30°, which is not suitable for the measurement of large-step microscopic surfaces, and the control of the silicon cantilever used in the probe needs to be realized by additional position detection systems such as optical lever method or optical interference method. , the leaked light generated by the optical detection system not only affects the measurement of the electrical parameters of the semiconductor device, but may also bring interference errors to the surface measurement. Therefore, the structure is complicated and adjustment and detection are difficult to realize. Therefore, it is not universally used in the existing surface topography measurement system.

Contents of the invention

In order to avoid the shortcomings of the above-mentioned prior art, the present invention provides a tap type high-sensitivity SPM probe and measurement method based on PVDF to achieve high spatial resolution, low measurement force, suitable for various materials, Measurement of surface topography that meets submillimeter microscopic height differences.

The present invention solves technical problem and adopts following technical scheme:

The structural characteristics of the tapping type high-sensitivity SPM measuring head based on PVDF of the present invention is to take the PVDF piezoelectric film as the vibrating beam and the tungsten probe as the scanning probe; The left and right ends of the beam are respectively fixed to the front ends of the opposite piezoelectric actuators on both sides through the clamping structure, and the rear ends of the piezoelectric actuators on both sides are respectively fixed to the respective cantilever beams. on the head frame; the scanning probe is fixedly arranged at the center of the lower surface of the vibrating beam.

The characteristic of the measuring method of the tapping type high-sensitivity SPM measuring head based on PVDF in the present invention is to apply an AC sinusoidal voltage signal with adjustable amplitude and frequency on the piezoelectric drivers on both sides as the driving signal. The driver produces back-and-forth displacement along the horizontal direction, and vibrates the scanning probe in the vertical direction through the vibrating beam; adjust the frequency of the driving signal to make the vibrating beam in a near-resonant state; during the scanning process, when the scanning probe has not touched the sample surface, The vibrating beam works in a near-resonance state, generating free amplitude A ₀ , and the output of the vibrating beam under the free amplitude A ₀ is the free amplitude charge C ₀ ; for the momentary contact between the scanning probe and the sample at a certain point, the vibrating beam The attenuation amplitude A ₁ is generated, and the output of the vibrating beam under the attenuation amplitude A ₁ is the attenuation charge C ₁ , the change of the charge output by the vibrating beam is used to represent the change of the vibration amplitude of the vibrating beam, and multi-point micro-measurement is carried out on the measured surface Force-tapping scanning enables measurement of the sample surface.

The PVDF (Polyvinylidene Fluoride) piezoelectric film used in the present invention has another name called polyvinylidene fluoride film, is a piezoelectric film made of high molecular piezoelectric material, has excellent piezoelectric properties, along the direction perpendicular to the polarized surface The electrical constant g ₃₁ can reach 0.26V·m/N. The density of the PVDF piezoelectric film is between 1.76 and 1.80g/cm ³ , it is light, thin, soft, and has certain elasticity. The scanning probe in the present invention is a tungsten needle tip with a large aspect ratio obtained by an electrochemical grinding method.

The present invention uses polyvinylidene fluoride piezoelectric film PVDF to replace the silicon material cantilever in the tapping mode atomic force microscope, and combines the tungsten probe in the scanning tunneling microscope to form a scanning probe measuring head. In this structure, it has piezoelectric effect The piezoelectric thin film PVDF is used not only as a microcantilever with a probe in an atomic force microscope (AFM), but also as a force-sensitive sensor in a scanning force microscope (SFM) to detect changes in small forces or displacements. Combining the measuring head of the present invention with the atomic force microscope measurement system can realize the detection of samples made of various materials (such as the surface of biological materials such as protein molecules), samples with steep step surface features (such as semiconductor silicon micro-devices, micro-electromechanical devices, etc.) equipment, etc.) for non-destructive microscopic surface topography measurements at nanometer resolution.

Compared with the prior art, the beneficial effects of the present invention are reflected in:

1. In the present invention, the piezoelectric driver is used to vibrate the PVDF piezoelectric film vibrating beam, and the PVDF piezoelectric film vibrating beam is used as the cantilever of the fixed scanning probe and also as a sensor for detecting the amplitude change of the PVDF piezoelectric film vibrating beam, so as to avoid The complicated additional position detection device is eliminated;

2. During the measurement process of the present invention, the PVDF vibrating beam works in a resonant state, and the scanning probe contacts the surface of the sample to be tested instantaneously, and the measurement force can be reduced to the nanonewton (10 ^-9 Newton) level, which can realize the protection against fragile damage The high-precision measurement of silicon micro-devices and nano-devices is more significant for the high-precision non-destructive measurement of thin, soft and fragile biological samples;

3. The present invention uses a tungsten probe with a large length-to-diameter ratio and a tip cone angle of about 10° as the scanning probe, so it is especially suitable for high-precision and micro-measurement force measurements on soft material surfaces and large-step surfaces.

Description of drawings

Fig. 1 is a schematic diagram of the PVDF membrane vibrating beam of the present invention.

Fig. 2 is a schematic diagram of the structure of the invention.

FIG. 3 is a schematic top view of the structure in FIG. 2 .

Labels in the figure: 1 Probe frame, 2 Adjustment groove, 3 Cantilever beam, 4 Piezoelectric driver, 5 Left piezoelectric driver drive line, 6 Vibration beam, 7 Scanning probe, 8 Film charge signal lead-out line, 9 Right side The piezoelectric driver drives the wire, and 10 clamping structures.

Below by way of embodiment, in conjunction with accompanying drawing, the present invention will be further described:

Detailed ways:

Referring to Fig. 1 and Fig. 2, in this example, the PVDF piezoelectric film is used as the vibrating beam 6, and the tungsten probe is used as the scanning probe 7; The clamping structures 10 are respectively fixed to the front ends of the opposite piezoelectric actuators 4 on both sides, and the rear ends of the piezoelectric actuators 4 on both sides are respectively fixed to the respective cantilever beams 3. On the head frame 1 ; the scanning probe 7 is fixedly arranged at the center of the lower surface of the vibrating beam 6 .

The measurement method is to apply an AC sinusoidal voltage signal with adjustable amplitude and frequency on the piezoelectric actuators 4 on both sides as the driving signal. Under the driving signal, the piezoelectric actuators 4 on both sides generate reciprocating displacement along the horizontal direction, and make The scanning probe 7 vibrates in the vertical direction; the frequency of the driving signal is adjusted so that the vibrating beam 6 is in a near-resonant state; during the scanning process, when the scanning probe 7 has not touched the sample surface, the vibrating beam 6 works in a near-resonant state, resulting in Free amplitude A ₀ , the output of the vibration beam 6 under the free amplitude A ₀ is the free amplitude charge C ₀ ; for the momentary contact between the scanning probe 7 and the sample at a certain point, the vibration beam 6 produces an attenuation amplitude A ₁ , The output of the vibrating beam under the attenuation amplitude A ₁ is the attenuation charge C ₁ , and the change of the vibration amplitude of the vibrating beam is characterized by the change of the output charge of the vibrating beam, and the multi-point micro-force tapping scanning is performed on the measured surface to realize the Measurement of the sample surface.

The adjustment slot 2 shown in Fig. 2 can be used to adjust the distance between the piezoelectric actuators 4 on both sides to achieve the purpose of adjusting the arc size of the PVDF film vibrating beam. The left piezoelectric actuator lead 5 and the right piezoelectric actuator The lead wire 9 is used as the input lead wire of the piezoelectric driver driving signal, and the thin film charge signal lead wire 8 is used as the lead end line of the charge signal.

About probe vibration frequency:

In the tap scanning probe based on PVDF vibrating beam, the principle of the vibrating beam structure is shown in Figure 1. In the figure, a layer of PVDF piezoelectric film constitutes the vibrating beam structure with a length of l. When the two ends are subjected to the force F of extrusion and stretching, forced vibration along the X direction will occur. According to the correlation analysis, the vibration frequency of this vibrating beam structure with both ends fixed can be expressed by the following formula:

$\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&pi;<figure-callout\; id="2"\; label="l"\; filenames="A200810156821E00101.png"\; state="\{\{state\}\}">l</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; EI</span>}}{\mathrm{<span\; class="notranslate">\; A\&rho;</span>}}}<span\; class="notranslate">\; \&times;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 500.56</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 12.302</span>}\frac{\mathrm{<span\; class="notranslate">\; f</span>}}{\mathrm{<span\; class="notranslate">\; EI</span>}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; l</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>$

Among them, A is the cross-sectional area of the PVDF piezoelectric film; F is the force loaded on the left and right ends of the piezoelectric film; l, b, h are the length, width and thickness of the piezoelectric film; E is Young's modulus of elasticity , I is the moment of inertia of the piezoelectric film.

The force F loaded on the left and right ends of the piezoelectric film can be expressed by the following formula:

F＝4πEI/l ₂

Then, the vibration frequency of the vibration beam structure composed of PVDF piezoelectric film along the X direction can be approximated by the following formula:

$\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; \&ap;</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&pi;<figure-callout\; id="2"\; label="l"\; filenames="A200810156821E00101.png"\; state="\{\{state\}\}">l</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; EI</span>}}{\mathrm{<span\; class="notranslate">\; A\&rho;</span>}}}<span\; class="notranslate">\; \&times;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 500.56</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 12.302</span>}\frac{\mathrm{<span\; class="notranslate">\; f</span>}}{\mathrm{<span\; class="notranslate">\; EI</span>}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; l</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&ap;</span>\frac{\mathrm{<span\; class="notranslate">\; 665</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="l"\; filenames="A200810156821E00101.png"\; state="\{\{state\}\}">l</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; EI</span>}}{\mathrm{<span\; class="notranslate">\; A\&rho;</span>}}}$

It can be seen from the above that the factors affecting the vibration frequency of the vibrating beam mainly include: the length l, width b, thickness h, and moment of inertia I of the piezoelectric film, and the others are constants related to materials. Therefore, changing the size parameters of the piezoelectric film also changes the vibration frequency of the vibrating beam.

Parameters affecting probe performance:

Two important parameters to measure the performance of the SPM probe are the resolution that the probe can achieve in the vertical direction, and the measuring force of the probe on the surface of the sample during the scanning of the sample. The parameters that characterize the level of these two performance indicators are mainly the elastic constant k of the vibrating arm and the quality factor Q.

The mechanical elastic constant k refers to the force on the PVDF piezoelectric film vibrating beam 6 (that is, the measurement force when the probe is in instant contact with the sample surface in tapping mode) and the amount of elastic deformation produced on the film vibrating beam 6 The ratio of , the unit is "Newton/meter". This ratio is directly related to the sensitivity of the probe, the size of the measuring force and the number of charges of the generated piezoelectric signal. That is to say, when the probe is in contact with the surface instantaneously during measurement, the smaller the value of the mechanical elastic constant k, the smaller the measuring force can cause the piezoelectric film vibrating beam 6 to have a large deformation, and the piezoelectric film can also produce Sufficient charge changes are used for detection, so such a vibrating beam has high sensitivity, the required measurement force is small, and the charge signal changes in a large range; on the contrary, the larger the value of the mechanical elastic constant k, the greater the measurement force is required. Only when the piezoelectric thin film vibrating beam 6 has enough deformation can it produce enough charge change for detection, the vibrating beam formed has low sensitivity, and the measurement force on the sample is very large, and the change of the charge signal is also small. obvious.

The quality factor Q is a fundamental parameter reflecting the dynamic performance of the system. In the vibration system, the quality factor Q is defined as follows:

$\mathrm{<span\; class="notranslate">\; Q</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}\frac{\mathrm{<span\; class="notranslate">\; E.</span>}}{\mathrm{<span\; class="notranslate">\; \&Delta;E</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; \&Delta;\&omega;</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="0"\; label="f"\; filenames="A200810156821E00101.png"\; state="\{\{state\}\}">f</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; \&Delta;\; f</span>}}$

Among them, E is the total energy of the vibration system; ΔE is the energy lost in one cycle of the vibration system; f ₀ is the resonant frequency when the amplitude reaches the maximum; and f ₁ <f ₀ <f ₂ , f ₁ and f ₂ are respectively f The vibration frequency value corresponding to 0.707 times the maximum amplitude on both sides of ₀ . That is to say, in a vibrating system, the quality factor characterizes how much energy is lost during one vibration period of the system, the size of the system damping and the height of the vibration peak.

For the above measuring head, the quality factor Q mainly affects the performance of the following three aspects: namely, the resonance amplitude that the PVDF film vibrating beam 6 can achieve during the tap scanning process; The magnitude of the amplitude change; the length of time required for the vibration beam 6 to return to a stable vibration state. That is, the higher the quality factor, the greater the change in the amplitude of the thin-film vibrating beam 6 under the same contact force, and the higher the force sensitivity, but the longer it takes to return to a stable state; otherwise, the change in the amplitude is smaller. Small, the force sensitivity is low, but it is easier to maintain a stable vibration state.

Therefore, in order to make the system have higher resolution and smaller force, it is necessary to adjust the structural parameters that affect Q and k reasonably.

After determining the structural parameters of the PVDF film and building the entire system, the sensitivity that this probe structure can achieve can be actually tested. The test is realized by the following method: control the nano-transition stage to move in the vertical direction, and continuously approach the tungsten probe 7 with a certain step amount (such as 1nm), when the probe 7 is in instantaneous contact with the nano-transition stage at a certain point , the vibration amplitude of the thin film vibrating beam 6 will attenuate. Due to the reduction of the amplitude of the film, the amount of charge generated by the PVDF piezoelectric film will also decrease accordingly, and the amplitude of the voltage signal detected by the detection circuit will also decrease accordingly. The sensitivity of the probe structure can be calculated by recording the step amount and the decrease amount of the voltage signal when the touch occurs, and the unit is V/μm. The higher the sensitivity, the higher the spatial resolution in the vertical direction that can be achieved by the SPM scanning probe microscope system built by this probe structure; otherwise, the lower resolution can be achieved. In addition, sensitivity testing can also be used to verify whether the parameters of the selected film are appropriate.

Experimental data:

In the experimental measurement, by changing the structural parameters of the PVDF film vibrating beam 6 such as the length l, width b and thickness h of the film and the radian of the vibrating beam 6, the performance that this new type of measuring head can achieve is measured. The vibrating beam 6 presents The radian can be realized by adjusting the distance D between the piezoelectric actuators 4 on both sides by adjusting the groove 2 .

The structural dimensions of the piezoelectric film determined from the experimental results are: l=10mm, b=2.5mm, h=40μm, and the radian of the vibrating beam is optimal when the distance between the clamping structures 10 is D=9.5mm. The quality factor Q of the probe is 24.3, and the elastic constant k is 35N/m.

Among them, the quality factor Q is much lower than the quality factor of the AFM probe (usually thousands), which is related to the structure of this new probe and the working environment under the atmosphere, resulting in a large damping of the vibration system, and the system The energy lost by vibration is also large. But in the case of ensuring sufficient sensitivity, the lower Q value can ensure the stability of the system vibration and shorten the time to return to the stable state, so that the scanning speed and work efficiency can be improved; and the mechanical elastic constant k of the vibrating beam is 35N/m, which is much smaller than the elastic constant of hundreds of N/m of the silicon microcantilever, which improves the sensitivity of this new type of measuring head and has a lighter measurement force on the surface of the sample.

It is determined by experiments that the scanning probe microscope system composed of this measuring head has a sensitivity of 1.98V/μm, a free vibration amplitude of 150-200nm, a vertical resolution of 1.6nm, and a scanning measurement force of nN level.

Claims (2)

1, based on PVDF tapping type high-sensitivity SPM measuring head, it is characterized in that be vibrating beam (6) with PVDF piezoelectric film, be scanning probe (7) with tungsten probe; Described vibrating beam (6) is set to A simply supported beam with a curved radian, the left and right ends of the vibrating beam (6) are respectively fixed to the front ends of the opposite piezoelectric actuators (4) on both sides through the clamping structure (10), and the piezoelectric actuators (4) on both sides The rear ends are respectively fixed on the respective cantilever beams (3), and the cantilever beams (3) are suspended on the probe frame (1) at one end; the scanning probes (7) are fixed on the vibrating beam (6) at the center of the lower surface. 2. The measuring method of the tap type high-sensitivity SPM measuring head based on PVDF is characterized in that an AC sinusoidal voltage signal with adjustable amplitude and frequency is applied on the piezoelectric drivers (4) on both sides as the drive signal, and the Under the driving signal, the piezoelectric actuators (4) on both sides produce back-and-forth displacement along the horizontal direction, and make the scanning probe (7) vibrate in the vertical direction through the vibrating beam (6); adjust the frequency of the driving signal so that the vibrating beam (6) is at Near-resonant state; during the scanning process, when the scanning probe (7) has not touched the _{sample surface, the vibrating beam (6) works in a near-resonant state, generating free amplitude A 0 ,} and the vibrating beam (6 ) is the free amplitude charge C ₀ ; for the momentary contact between the scanning probe (7) and _the sample at a certain point, the vibrating beam (6) produces an attenuated amplitude A ₁ , and the vibrating beam ( 6) The output is the attenuated charge C ₁ , the change of the charge output by the vibrating beam (6) is used to represent the change of the vibration amplitude of the vibrating beam, and the multi-point micro-measurement force is tapped and scanned on the surface to be tested to realize the test measurement of the sample surface.

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