A new type of quartz crystal microbalance with concave ring structure

Abstract

The present invention discloses a novel quartz crystal microbalance with a concave platform annular structure, comprising a quartz wafer with a concave platform, metal electrodes attached to the upper and lower surfaces, the lower surface being a circular electrode, and the upper surface being an annular electrode; the lower surface of the quartz wafer is flat, and an anti-table is etched in the central area of ​​the upper surface, the radius of the anti-table is greater than the radius area of ​​the circular electrode, and the surrounding area remains unchanged. The present invention improves the structure of the annular quartz crystal microbalance, and solves the problem of the absolute mass sensitivity decrease of the annular electrode QCM when the mass sensitivity is homogenized by etching a groove. This special design not only reduces the repeatability error, but also improves the detection accuracy.

Description

Technical Field

[0001] The invention belongs to the technical field of sensor detection, in particular to a novel quartz crystal microbalance with a concave platform ring structure. Background Art

[0002] Quartz Crystal Microbalance (QCM) technology is a new type of sensor measurement technology established in the 1960s. It is a piezoelectric sensor with high resolution formed by plating circular gold electrodes on the upper and lower surfaces of AT-cut quartz wafers. Figure 1 As shown, the piezoelectric effect is used to convert the mass change information on its surface into the information of the resonant frequency change, that is, to convert the physical information of mass into electrical signal information, so as to facilitate the use of various signal analysis and processing technologies to analyze and process the physical signal, and then obtain the information we need. The quartz crystal microbalance has very high detection accuracy and can perform mass detection in the nanogram level (ng). As a new type of sensor, QCM sensors have made great progress in recent years. The quartz crystal microbalance has the advantages of simple structure, high sensitivity, low cost, easy real-time online detection and high degree of digitization. Its application fields are becoming more and more extensive, gradually covering medical diagnosis, electrochemical analysis, biomedicine, food analysis and other disciplines.

[0003] In recent years, QCM mainly considers two indicators. On the one hand, it is the uniformity of mass sensitivity, that is, reducing the repeatability error. On the other hand, it is the value of mass sensitivity, that is, the detection accuracy. The mass sensitivity distribution function of QCM is defined as follows

[0004]

[0005] Where A(r) is the particle displacement amplitude function, r is the distance from the particle to the electrode center, C f is the mass sensitivity constant, S f (r) is the mass sensitivity distribution function.

[0006] The mass sensitivity of the traditional circular electrode QCM presents a Gaussian distribution, which decays continuously on both sides of the peak value, so the repeatability of the detection results is relatively poor. Since the shape, thickness, size and material of the metal electrode of the QCM have a great influence on the mass sensitivity, in order to solve the problem of uneven distribution of the mass sensitivity of the circular electrode QCM, Josse and Lee proposed a QCM mass sensor in the shape of a ring electrode. Its mass sensitivity distribution curve is a bimodal curve, but different load factors have a great influence on the uniformity of the curve. Richardson et al. achieved a more uniform mass sensitivity distribution by adjusting the mass load factor of the ring electrode. Many domestic researchers have made great improvements in the uniformity of QCM through different electrode structure designs (multi-ring, point ring, ellipse). When the electrode structure changes from circular to ring-shaped, the curve changes from Gaussian distribution to a bimodal distribution with a depression, the repeatability of the detection results will be relatively better, and the uniformity of the mass sensitivity will be improved, but at the same time it will bring about the attenuation of mass sensitivity and reduce the accuracy of detection, such as Figure 2 shown. Summary of the invention

[0007] The technical problem solved by the present invention is to provide a new type of quartz crystal microbalance with a concave ring structure that improves the absolute mass sensitivity while maintaining uniformity, in order to address the problem of mass sensitivity attenuation of the existing quartz crystal microbalance mass sensor with a ring electrode structure. This improves the uniformity and detection accuracy of the QCM and comprehensively improves the detection performance of the QCM.

[0008] The technical solution of the present invention to achieve the above technical objectives is as follows: a new type of quartz crystal microbalance with a concave platform ring structure, including a quartz wafer with a concave platform, metal electrodes attached to the upper and lower surfaces, the lower surface is a circular electrode, and the upper surface is a ring electrode.

[0009] In one embodiment, the lower surface of the quartz wafer is flat, and an anti-table is etched in the central area of ​​the upper surface. The radius of the anti-table is larger than the radius of the circular electrode area and remains unchanged around it.

[0010] In one embodiment, the upper and lower surface electrodes are made of gold with a thickness of h e The diameter of the circular electrode is 100nm, the radius of the circular electrode is 3mm, and the inner and outer radii of the annular electrode are a:b=1:4.

[0011] In one embodiment, the radius of the quartz wafer is 7 mm, and the thickness h of the quartz wafer is 0.272 mm.

[0012] In one embodiment, the quartz wafer used is an AT-cut quartz wafer.

[0013] In one embodiment, the resonant frequency of the quartz crystal microbalance mass sensor is 6 MHz, and the working overtone number is the fundamental frequency.

[0014] In one embodiment, the etching thickness is 54 um to 72 um.

[0015] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention improves the structure of the annular quartz crystal microbalance and solves the problem of the absolute mass sensitivity of the annular electrode QCM decreasing when the mass sensitivity is homogenized by etching a groove. This special design reduces the repeatability error and improves the detection accuracy. (2) The annular electrode is attached to the concave quartz crystal. A thinner quartz crystal has a higher resonant frequency, which can improve the detection accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a diagram of the structure of a QCM with a circular electrode shape.

[0017] Figure 2 This is a comparison of the mass sensitivity curves of QCMs with circular and ring electrode shapes.

[0018] Figure 3 It is the structural diagram of the concave ring electrode QCM.

[0019] Figure 4 It is a comparison of the mass sensitivity curves of the ring electrode and the concave ring electrode QCM.

[0020] Figure 5 This is a comparison of the QCM mass sensitivity curves of concave ring electrodes etched with different heights.

[0021] Figure 6 It is a comparison chart of coefficient of variation at different heights.

[0022] Figure 7 This is a comparison chart of mass sensitivity at different heights. DETAILED DESCRIPTION

[0023] The present invention proposes a novel quartz crystal microbalance with a concave platform annular structure, which increases the absolute mass sensitivity while maintaining the uniformity of mass sensitivity; the present invention etches a groove on the quartz crystal, the detection electrode surface does not change, and the entire structure is divided into a full electrode area (E area), a partial electrode area (P area) and a non-electrode area (U area), h is the thickness of the quartz crystal, h′ is the height of the concave platform, and h q is the height of the quartz crystal in the concave region, h e is the thickness of the electrode, and a and b are the distances from the inner and outer sides of the annular electrode to the center of the electrode.

[0024] QCM sensor with concave ring design, such as Figure 3 As shown, the upper and lower electrode surfaces are circular and annular respectively, and the materials are both gold, wherein the circular electrode is the detection electrode; the non-detection electrode, i.e., the annular electrode is attached to the concave quartz crystal. The thinner quartz crystal has a higher resonance frequency, which can improve the detection accuracy.

[0025] The structural design provided by the present invention combines the concave platform with the existing ring electrode QCM sensor. The design is based on the piezoelectric effect and thickness shear mode, and also meets the mechanical boundary conditions and electrical boundary conditions. The thickness shear acoustic wave can be expressed as

[0026]

[0027] Where A is the particle vibration displacement, is the wave number of the excitation frequency, is the wave number of the cutoff frequency, ω and ω c are the excitation angular frequency and the cutoff angular frequency respectively, v is the propagation wave velocity in the crystal, and the directions of x1 and x3 are as follows: Figure 3 As shown. In formula (2), the values ​​of anisotropy constants a1 and a3 depend on the material of the crystal. From this, the shear horizontal acoustic wave number k in the crystal thickness direction can be obtained. r :

[0028]

[0029] where f ce ,f cp ,f cu denote the resonant frequencies of the full electrode region, partial electrode region, and non-electrode region, respectively; h is the height of the concave terrace region; and f0 is the fundamental frequency. They represent the shear level acoustic wave number of the full electrode area, partial electrode area and non-electrode area respectively. According to the energy trap theory, when the acoustic wave propagates on the chip, there is a minimum frequency called the cutoff frequency. Only when the operating frequency is higher than the cutoff frequency can the acoustic wave propagate freely on the chip and may produce standing waves and resonance. The acoustic wave below this frequency cannot propagate and shows exponential decay around the wave source. The vibration energy is confined to the electrode area. The displacement amplitude function A(r) of the particle is obtained as follows:

[0030]

[0031] where J0 and N0 are zero-order Bessel functions of the first and second kind, I0 and K0 are zero-order modified Bessel functions of the first and second kind, C1, C2, C3, and C4 are unknown amplitude constants obtained by solving a system of linear equations, r is the distance from the center of the quartz crystal, and a and b are as follows: Figure 3As shown in the figure, they are the distances from the inner and outer edges of the annular electrode to the center of the quartz crystal. The displacement amplitude function A(r) of the particle is known, and the mass sensitivity distribution function S can be obtained by integration. f (r).

[0032] By attaching Figure 4 To the attached Figure 6 It can be seen that the electrode structure design proposed in the present invention can maintain the uniformity of mass sensitivity and the repeatability of QCM measurement results, while improving the absolute mass sensitivity, thereby achieving the purpose of improving QCM measurement accuracy.

[0033] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] Example

[0035] The QCM of the concave ring electrode is as follows Figure 3 As shown, it includes a quartz crystal with a single-sided depression and metal electrodes attached to the upper and lower surfaces of the quartz crystal, which are divided into three areas, namely the full electrode area (E area), the partial electrode area (P area) and the non-electrode area (U area). The upper surface, that is, the surface electrode structure of the etched quartz crystal is annular, and the lower surface electrode structure is circular, which serves as a detection electrode. The metal electrode material of the invention is gold with a thickness of 100nm. The maximum radius of the quartz crystal is 7mm, the thickness at the outer circle is 0.272mm, the radius of the circular electrode is 3mm, and the radii of the inner and outer parts of the annular electrode are 0.75mm and 3mm respectively. AT-cut quartz crystal is used, and the resonant frequency of the QCM sensor is 6MHz, working at the fundamental frequency.

[0036] According to the above scheme, the designed 6MHz base frequency AT-cut QCM quality sensor is theoretically calculated and simulated. Figure 4 As shown in the figure, the QCM comparison of the concave ring and the ring electrode, the mass sensitivity curve shows that the concave ring greatly improves the absolute mass sensitivity while maintaining uniformity, indicating the effectiveness of the concave ring design. Due to the different etching heights, that is, the different heights of the concave area, it has a certain impact on the resonance, so multiple groups of heights were designed for verification, which are 1-6, and the etching height gradually deepens. Figure 5 As shown, as the etching height increases, the curve depression in some electrode areas also gradually deepens, so the thickness of the quartz crystal cannot be too small.

[0037] Since the repeatability of QCM is closely related to the uniformity of the sensitivity distribution on the electrode surface, we use the coefficient of variation (c v ) is used to represent the uniformity of the mass sensitivity distribution in the electrode area, where the coefficient of variation is the standard deviation of the sensitivity data on the measured electrode surface divided by its mean value. Figure 6The coefficient of variation line graph of sensitivity data under 6 heights is given. Figure 6 It can be seen that the coefficient of variation shows a trend of first decreasing and then increasing, which is also in line with Figure 5 The regularity of the curve also verifies that the QCM with concave ring electrode can maintain the uniformity of mass sensitivity and the repeatability of detection.

[0038] The concave ring electrode QCM designed according to the above specifications and parameters simulates the actual effects under 6 sets of etching heights, among which the mass sensitivity change trend is as follows Figure 7 As shown. Figure 7 In the study, the absolute mass sensitivity is proportional to the increase in etching height, which is consistent with the research rule that the thinner the quartz crystal, the higher the resonant frequency.

[0039] The results show that the QCM mass sensor with concave-platform annular electrode provided by the present invention improves the absolute mass sensitivity and the detection accuracy while maintaining a uniform mass sensitivity distribution.

Claims

1. A novel quartz crystal microbalance with a concave ring structure, characterized in that: It includes a quartz wafer with a concave terrace, metal electrodes attached to the upper and lower surfaces, the lower surface is a circular electrode, and the upper surface is an annular electrode; the lower surface of the quartz wafer is flat, and an anti-terrace is etched in the central area of ​​the upper surface, and the radius of the anti-terrace is larger than the radius area of ​​the circular electrode; the inner and outer radii of the annular electrode a:b=1:

4.

2. The novel quartz crystal microbalance with concave platform annular structure according to claim 1 is characterized in that: The upper and lower surface electrodes are made of gold.

3. The novel quartz crystal microbalance with concave platform ring structure according to claim 2 is characterized in that: The thickness of the upper and lower surface electrodes is 100nm.

4. The novel quartz crystal microbalance with concave platform ring structure according to claim 3 is characterized in that: The radius of the circular electrode is 3 mm.

5. The novel quartz crystal microbalance with concave platform ring structure according to claim 1 is characterized in that: The radius of the quartz wafer is 7 mm, and the thickness h of the quartz wafer is 0.272 mm.

6. The novel quartz crystal microbalance with concave platform ring structure according to claim 5 is characterized in that: The quartz wafer is an AT-cut quartz wafer.

7. The novel quartz crystal microbalance with concave platform ring structure according to claim 3 is characterized in that: The resonant frequency of the quartz crystal microbalance mass sensor is 6MHz, and the working overtone number is the fundamental frequency.

8. The novel quartz crystal microbalance with concave platform ring structure according to claim 1 is characterized in that: The etching thickness is 54um~72um.

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