Structure for reducing squid superconducting loop area, and preparation method therefor

Abstract

Provided are a structure for reducing a SQUID superconducting loop area, and a preparation method therefor, wherein a stacked structure is formed on a silicon substrate, and precision etching is performed on the stacked structure, so that a 3D nano-bridge junction is formed on a sloped surface of the stacked structure and stretches across an insulating layer, such that batch preparation of the structure for reducing a SQUID superconducting loop area can be implemented, which facilitates large-scale application of SQUID probes. The plane of the 3D nano-bridge junction is perpendicular to the plane on which a SQUID superconducting loop is located, such that the width of the 3D nano-bridge junction no longer affects a SQUID effective loop area, so that the thickness of the 3D nano-bridge junction can be individually regulated in order to improve SQUID performance, which significantly weakens the inhibition effect of a magnetic field on the superconducting characteristics of the 3D nano-bridge junction. In addition, the SQUID superconducting loop having a minimum line width of 10nm can be prepared in combination with an electron beam lithography technique having higher precision, significantly reducing the SQUID superconducting loop area, improving spin sensitivity, and thereby achieving higher spatial resolution during scanning magnetic imaging.

Description

A structure for reducing the area of ​​a SQUID superconducting loop and its fabrication method Technical Field

[0001] This invention relates to the field of superconducting quantum interference devices (SQUIDs), and in particular to a structure for reducing the superconducting loop area of ​​a SQUID and its fabrication method. Background Technology

[0002] With the development of spintronics and superconducting electronics, studying the direct magnetic response of materials at the microscale can reveal properties that many other methods cannot detect, such as the observation of magnetization relaxation processes in nanoparticles, magnetic flux imaging in nanowires, and magnetic flux quantization and vortex detection in superconductors. The DC SQUID (Superconducting quantum interference device), composed of two Josephson junctions (JJs) connected in parallel, is one of the most sensitive known magnetic flux sensors, with a detection sensitivity approaching the quantum limit. In microscopic magnetic imaging, the SQUID probe paired with a high-precision triaxial scanning platform (SSM) enables submicron-level magnetic flux imaging due to its high magnetic field sensitivity and non-destructive readout via weak magnetic coupling.

[0003] The sensitivity and resolution of scanning SQUID microscopy are directly related to the size of the SQUID superconducting loop. Compared to traditional micron-scale SQUID superconducting loops, SQUID probes with nanoscale superconducting loops have smaller loop sizes, higher imaging resolution, lower magnetic flux noise, and higher spin sensitivity.

[0004] While nano-SQUIDs based on thin-film bridged Josephson junctions can easily realize nanoscale SQUID superconducting loop structures, such as SQUID on-Title structures fabricated on the tip of a quartz glass tube, achieving magnetic imaging resolution of around 50 nm, the contact effect of large-area electrodes of equal thickness at both ends of the nanobridge causes superconducting phase interference to diffuse to the electrode location. This weakens the Josephson effect at the connection between the nanobridge and the large-area electrodes and the voltage-flux modulation performance of the SQUID. Secondly, since the plane of the nanobridge film is on the same plane as the SQUID superconducting loop, the magnetic field of the SQUID superconducting loop suppresses the superconductivity of the nanobridge, thus reducing the maximum operating magnetic field of the SQUID probe. Furthermore, in terms of fabrication, SQUID on-Title structures on quartz tubes cannot be mass-produced, hindering large-scale applications. Furthermore, nano-SQUID probes based on nanobridges, which can be mass-produced on silicon wafers, also face the same problem: the plane containing the SQUID superconducting loop and the plane containing the superconducting nanobridge film lie on the same plane. Due to the influence of the superconducting film's penetration depth, the effective loop diameter of conventional SQUID superconducting loops is mostly above 600 nm, making the effective area for trapping magnetic flux in the SQUID superconducting loop much larger than its physical geometric area, thus limiting the spatial resolution of the SQUID probe.

[0005] It should be noted that the above introduction to the technical background is only for the purpose of providing a clear and complete explanation of the technical solutions of this application and facilitating understanding by those skilled in the art. It should not be assumed that these technical solutions are known to those skilled in the art simply because they have been described in the background section of this application. Summary of the Invention

[0006] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a structure and preparation method for reducing the area of ​​the SQUID superconducting loop, which solves the problem that the maximum working magnetic field of the SQUID probe is reduced in the prior art, and the effective loop diameter of the conventional SQUID superconducting loop is mostly above 600 nm due to the influence of the magnetic field on the penetration depth of the superconducting thin film. This results in the effective area of ​​the SQUID superconducting loop for capturing magnetic flux being much larger than the physical geometric area, thus limiting the spatial resolution of the SQUID probe.

[0007] To achieve the above and other related objectives, the present invention provides a structure for reducing the area of ​​a SQUID superconducting loop, the structure comprising:

[0008] A silicon substrate, on which a silicon dioxide thin film is formed, and on which an isosceles trapezoidal stacked structure is formed, the stacked structure comprising, from bottom to top, a first superconducting thin film layer, an insulating layer, and a second superconducting thin film layer, wherein both sidewalls of the stacked structure form slopes with an inclination angle θ with the silicon substrate;

[0009] A third superconducting thin film layer is disposed on the silicon substrate and the slope near the stacked structure and is connected to the first superconducting thin film layer and the second superconducting thin film layer, respectively.

[0010] 3D nanobridges are symmetrically arranged on the slope and distributed across the insulating layer, and the insulating layer between the 3D nanobridges forms a SQUID superconducting loop.

[0011] Optionally, the tilt angle θ ranges from 45° to 85°.

[0012] Optionally, the geometric height of the SQUID superconducting loop is 5–20 nm, which is the thickness of the insulating layer structure, and the geometric width is 10–100 nm, which is the width of the insulating layer structure.

[0013] Optionally, the plane containing the 3D nanobridge is perpendicular to the plane containing the SQUID superconducting loop.

[0014] Optionally, the first superconducting thin film layer, the second superconducting thin film layer, and the third superconducting thin film layer are made of the same material.

[0015] Optionally, the widths of the first superconducting thin film layer, the insulating layer, and the second superconducting thin film layer decrease sequentially.

[0016] Optionally, the thickness of the first superconducting thin film layer is 5–300 nm, the thickness of the second superconducting thin film layer is 5–300 nm, and the thickness of the third superconducting thin film layer is 5–30 nm.

[0017] Optionally, the insulating layer is made of one of SiO2, MgO, AlN, or Al2O3, and the thickness of the insulating layer is 5–20 nm.

[0018] This invention also provides a method for fabricating a reduced-area SQUID superconducting loop structure, comprising the following steps:

[0019] A silicon substrate is provided, a silicon dioxide thin film is formed on the silicon substrate, and a stacked structure is formed on the silicon dioxide thin film, the stacked structure comprising a first superconducting thin film layer, an insulating layer and a second superconducting thin film layer deposited sequentially.

[0020] The stacked structure is exposed and subjected to reactive ion beam etching, so that the shape of the stacked structure is an isosceles trapezoid, and both sidewalls of the stacked structure form slopes with an inclination angle θ with the silicon substrate.

[0021] A third superconducting thin film layer is deposited again on the slope and the silicon substrate near the stacked structure. Electron beam lithography is performed on the third superconducting thin film layer. By controlling the different exposure doses, 3D nanobridges are formed on the slope of the insulating layer. The 3D nanobridges are symmetrically arranged on both sides of the slope of the stacked structure.

[0022] Optionally, the materials of the first superconducting thin film layer, the second superconducting thin film layer, and the third superconducting thin film layer include one of Nb, NbN, Al, and Pb.

[0023] As described above, the present invention proposes a structure and fabrication method for reducing the area of ​​a SQUID superconducting loop. Compared with existing technologies, the SQUID probe is fabricated on a silicon substrate, enabling mass production and facilitating large-scale applications. The thickness of the third superconducting film at both ends of the 3D nanobridge in the SQUID probe is greater than the thickness of the 3D nanobridge, further enhancing the performance of the SQUID probe. The plane of the 3D nanobridge is perpendicular to the plane of the SQUID superconducting loop, thereby reducing the area surrounded by the superconducting film magnetic field and decreasing the effective area of ​​the SQUID superconducting loop. The width of the 3D nanobridge is no longer a limiting factor for the effective area of ​​the SQUID superconducting loop; therefore, the area of ​​the SQUID superconducting loop can be adjusted by changing the thickness of the insulating layer, thus optimizing the performance of the SQUID probe. Furthermore, this method, combined with electron beam lithography, a technique with higher etching precision, can fabricate SQUID superconducting loops with a minimum linewidth of 10 nm, significantly reducing the SQUID superconducting loop area and improving spin sensitivity, thereby achieving higher spatial resolution in scanning magnetic imaging. Attached Figure Description

[0024] Figure 1 shows a schematic diagram of the structure of the present invention that reduces the area of ​​the SQUID superconducting loop.

[0025] Figure 2 shows a front view of the structure of the present invention with reduced SQUID superconducting loop area.

[0026] Figure 3 shows a schematic diagram of the SQUID superconducting loop formed in the structure of the present invention with reduced SQUID superconducting loop area.

[0027] Figure 4 shows a process flow diagram of the method for fabricating the reduced SQUID superconducting loop area structure of the present invention.

[0028] Figure 5 shows a schematic diagram of the structure in the method for preparing the reduced SQUID superconducting loop area structure of the present invention.

[0029] Component labeling descriptions: 101, silicon substrate; 102, first superconducting thin film layer; 103, insulating layer; 104, second superconducting thin film layer; 105, third superconducting thin film layer; 106, 3D nanobridge; 107, SQUID superconducting loop; S1~S3, steps. Detailed Implementation

[0030] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0031] It should be noted that the illustrations provided in this embodiment are only schematic representations of the basic concept of the present invention. Therefore, the illustrations only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0032] Please refer to Figures 1 to 5. It should be understood that the structures, proportions, sizes, etc., depicted in the accompanying drawings are merely for illustrative purposes to aid those skilled in the art and are not intended to limit the scope of the invention. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in proportions, or adjustments to size, without affecting the effectiveness and objectives of the invention, should still fall within the scope of the disclosed technical content. Furthermore, the terms "upper," "lower," "left," "right," "middle," "first," and "second," etc., used in this specification are merely for clarity and not intended to limit the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0033] As shown in Figures 1 to 3, this embodiment provides a structure that reduces the area of ​​the SQUID superconducting loop 107, the structure comprising:

[0034] A silicon substrate 101 is provided, on which a silicon dioxide thin film is formed. A stacked structure in the shape of an isosceles trapezoid is formed on the silicon dioxide thin film. The stacked structure includes, from bottom to top, a first superconducting thin film layer 102, an insulating layer 103, and a second superconducting thin film layer 104. Both sidewalls of the stacked structure form slopes with an inclination angle θ with the silicon substrate 101.

[0035] The third superconducting thin film layer 105 is disposed on the silicon substrate 101 near the stacked structure and on the slope surface, and is connected to the first superconducting thin film layer 102 and the second superconducting thin film layer 104 respectively.

[0036] 3D nanobridges 106 are symmetrically arranged on the slope and distributed across the insulating layer 103. The insulating layer 103 between the 3D nanobridges 106 forms a SQUID superconducting loop 107.

[0037] Furthermore, the structure proposed in this embodiment, which reduces the area of ​​the SQUID superconducting loop 107, can also be combined with deep silicon etching technology to etch away the silicon substrate 101 corresponding to the SQUID probe, thereby achieving precise control over the distance between the SQUID probe and the edge of the silicon wafer tip. This distance can be reduced from 2μm in the prior art to about 300nm, improving the coupling strength between the SQUID probe and the magnetic signal, and improving the intensity and spatial resolution of the scanning imaging signal.

[0038] As shown in Figure 2, as an example, the tilt angle θ ranges from 45° to 85°. The silicon substrate 101 includes a front side and a back side disposed opposite to each other. A silicon dioxide thin film is formed on the front side of the silicon substrate 101. A stacked structure is provided on the silicon dioxide thin film. The stacked structure includes, from bottom to top, a first superconducting thin film layer 102, an insulating layer 103, and a second superconducting thin film layer 104, with the widths of the first superconducting thin film layer 102, the insulating layer 103, and the second superconducting thin film layer 104 decreasing sequentially, making the stacked structure an isosceles trapezoid. During the fabrication of the stacked structure, the ratio of oxygen (O2) to carbon tetrafluoride (CF4) during the etching process is controlled to ensure that the tilt angle θ between the two sides of the stacked structure and the silicon substrate 101 meets the target requirements.

[0039] As shown in Figures 2 and 3, as an example, the geometric height of the SQUID superconducting loop 107 is 5–20 nm, and the geometric width is 10–100 nm. The 3D nanobridges 106 are symmetrically arranged on the slope and distributed across the insulating layer 103, such that the insulating layer 103 between the 3D nanobridges 106 on both side walls constitutes the SQUID superconducting loop 107. The minimum area of ​​the SQUID superconducting loop 107 mainly depends on the slope area of ​​the insulating layer 103 below the 3D nanobridges 106 and the effective magnetic field penetration depth λ. Specifically, the width of the SQUID superconducting loop 107 depends on the thickness of the insulating layer 103, which is between 5 and 20 nm, and the length of the SQUID superconducting loop 107 depends on the length of the insulating layer 103, which is between 10 and 100 nm. Compared with the traditional micron-scale SQUID superconducting loop, the SQUID superconducting loop 107 formed by the present invention is at the nanometer level, which greatly reduces the effective area of ​​the SQUID superconducting loop 107, thereby improving spin sensitivity and spatial resolution.

[0040] As shown in Figures 1 and 2, as an example, the first superconducting thin film layer 102, the second superconducting thin film layer 104, and the third superconducting thin film layer 105 are made of the same material. A third superconducting thin film layer 105 is also formed on the sidewall slopes of the stacked structure and on the silicon substrate 101 near the stacked structure. A 3D nanobridge 106 is formed in the middle of the third superconducting thin film layer 105 on the slope, with both ends of the 3D nanobridge 106 connected to the third superconducting thin film layer 105. The third superconducting thin film layer 105 on the slope has good electrical connection with the slopes of the first superconducting thin film layer 102 and the second superconducting thin film layer 104. Specifically, the materials of the first superconducting thin film layer 102, the second superconducting thin film layer 104, and the third superconducting thin film layer 105 are selected from Nb, NbN, Al, and Pb. Among them, NbN has a higher superconducting transition temperature and critical magnetic field. The superconducting critical temperature of NbN is 10.5–13.2 K, and that of Nb is 7.5–9.3 K. Of course, provided that the structural performance of reducing the area of ​​the SQUID superconducting loop 107 is met, the materials of the first superconducting thin film layer 102, the second superconducting thin film layer 104, and the third superconducting thin film layer 105 can be selected according to the actual situation, and no restrictions are imposed here.

[0041] As shown in Figure 2, as an example, the plane of the 3D nanobridge 106 is perpendicular to the plane of the SQUID superconducting loop. Specifically, when the structure with a reduced area of ​​the SQUID superconducting loop 107 is in working mode, based on the superconducting Josephson effect and magnetic flux quantization effect, the changing magnetic field (stray magnetic field on the sample surface) can be measured according to its magnetic field response curve. A Cartesian coordinate system as shown in Figure 2 is established. During testing, the direction of the magnetic field applied to the SQUID superconducting loop 107 is distributed along the y-axis, that is, the direction of the applied magnetic field is parallel to the plane of the 3D nanobridge 106. This prevents the applied magnetic field from passing perpendicularly through the plane of the 3D nanobridge 106, thereby reducing the inhibitory effect of the magnetic field on the superconducting properties of the 3D nanobridge 106 and making it possible for the structure to work under magnetic fields above 1T.

[0042] As shown in Figures 1 and 2, as an example, the thickness of the first superconducting thin film layer 102 is 5–300 nm, the thickness of the second superconducting thin film layer 104 is 5–300 nm, and the thickness of the third superconducting thin film layer 105 is 5–30 nm. In this structure with a reduced area of ​​the SQUID superconducting loop 107, the thickness of the third superconducting thin film at both ends of the 3D nanobridge 106 is greater than the thickness of the 3D nanobridge 106, which can obtain a superconducting current phase relationship that is closer to a sinusoidal function, thereby further improving the performance of the structure with a reduced area of ​​the SQUID superconducting loop 107.

[0043] As shown in Figure 2, the insulating layer 103 is made of one of SiO2, MgO, AlN, or Al2O3, and its thickness is 5–20 nm. Materials such as SiO2 and MgO have advantages such as good insulation and good sidewall encapsulation, which can reduce the penetration of superconducting materials and further reduce the effective area of ​​the SQUID superconducting loop 107.

[0044] As shown in Figures 4 and 5, as another example, the present invention also provides a method for fabricating a reduced-area SQUID superconducting loop 107 structure, comprising the following steps:

[0045] S1: A silicon substrate 101 is provided, a silicon dioxide thin film is formed on the silicon substrate 101, and a stacked structure is formed on the silicon dioxide thin film. The stacked structure includes a first superconducting thin film layer 102, an insulating layer 103 and a second superconducting thin film layer 104 deposited sequentially.

[0046] S2: Expose and perform reactive ion beam etching on the stacked structure to make the shape of the stacked structure an isosceles trapezoid, and the two side walls of the stacked structure form a slope with an inclination angle θ with the silicon substrate 101.

[0047] S3: A third superconducting thin film layer 105 is deposited again on the slope and the silicon substrate 101 near the stacked structure. Electron beam lithography is performed on the third superconducting thin film layer 105. By controlling the different exposure doses, 3D nanobridges 106 are formed on the slope of the insulating layer 103. The 3D nanobridges 106 are symmetrically arranged on both sides of the slope of the stacked structure.

[0048] Specifically, as shown in Figure 5a, a silicon dioxide thin film (not shown in the figure) is first formed on a silicon substrate 101 using a deposition process. Then, a first superconducting thin film layer 102 of a certain thickness is deposited on the silicon dioxide thin film, as shown in Figures 5b and 5c. Next, an insulating layer 103 and a second superconducting thin film layer 104 are deposited sequentially on the first superconducting thin film layer 102, thus obtaining a stacked structure. The deposition process can effectively control the thickness of each thin film layer, thereby controlling the effective area of ​​the SQUID superconducting loop 107. As shown in Figure 5d, the stacked structure is exposed and subjected to reactive ion beam etching. By controlling the ratio of oxygen (O2) and carbon tetrafluoride (CF4) during the reactive ion beam etching process, the etching area on both sides of the stacked structure can be changed, allowing both sides of the stacked structure to form a slope with an angle θ with the silicon substrate 101.

[0049] As shown in Figure 5e, a third superconducting thin film layer 105 is deposited again on the slope and the silicon substrate 101 using a deposition process. The third superconducting thin film layer 105 has a good electrical connection with the slope of the first superconducting thin film layer 102 and the second superconducting thin film layer 104. Then, using electron beam lithography, which has higher etching precision, and by controlling the exposure dose, the third superconducting thin film layer 105 forms 3D nanobridges 106 on the slope of the insulating layer 103. The formed 3D nanobridges 106 are symmetrically distributed on both sides of the slope of the stacked structure.

[0050] As an example, the materials of the first superconducting thin film layer 102, the second superconducting thin film layer 104, and the third superconducting thin film layer 105 include one of Nb, NbN, Al, and Pb. Niobium nitride has a higher superconducting transition temperature and critical magnetic field; the superconducting critical temperature of niobium nitride is 10.5–13.2 K, and that of niobium is 7.5–9.3 K. Of course, while meeting the structural performance requirements of reducing the area of ​​the SQUID superconducting loop 107, the materials of the first superconducting thin film layer 102, the second superconducting thin film layer 104, and the third superconducting thin film layer 105 can be selected according to actual conditions, and no restrictions are imposed here.

[0051] In summary, the structure and fabrication method for reducing the superconducting loop area of ​​SQUID according to the present invention, compared with the prior art, is based on forming a stacked structure on a silicon substrate, allowing 3D nanobridges to be formed on the slope of the stacked structure and spanning the insulating layer. This enables the mass production of structures with reduced SQUID superconducting loop area, which is more conducive to the large-scale application of SQUID probes. In addition, the superconducting film thickness at both ends of the 3D nanobridges in the structure with reduced SQUID superconducting loop area is greater than that of the 3D nanobridges themselves, further improving the performance of SQUID. The plane of the 3D nanobridges is aligned with the SQUID superconducting... The plane of the loop is perpendicular, thus reducing the area surrounded by the magnetic field of the superconducting thin film and decreasing the effective area of ​​the SQUID superconducting loop. The width of the 3D nanobridge is no longer a limiting factor for the effective area of ​​the SQUID superconducting loop. Therefore, the area of ​​the SQUID superconducting loop can be adjusted by adjusting the thickness of the insulating layer, thereby optimizing the performance of the SQUID probe. In addition, by combining it with the more precise electron beam lithography technology, SQUID superconducting loops with a minimum linewidth of 10 nm can be fabricated, which greatly reduces the area of ​​the SQUID superconducting loop and improves the spin sensitivity, thereby achieving higher spatial resolution in scanning magnetic imaging.

[0052] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A structure for reducing the area of ​​a SQUID superconducting loop, characterized in that, The structure includes: A silicon substrate, on which a silicon dioxide thin film is formed, and on which an isosceles trapezoidal stacked structure is formed, the stacked structure comprising, from bottom to top, a first superconducting thin film layer, an insulating layer, and a second superconducting thin film layer, wherein both sidewalls of the stacked structure form slopes with an inclination angle θ with the silicon substrate; A third superconducting thin film layer is disposed on the silicon substrate and the slope near the stacked structure and is connected to the first superconducting thin film layer and the second superconducting thin film layer, respectively. 3D nanobridges are symmetrically arranged on the slope and distributed across the insulating layer, and the insulating layer between the 3D nanobridges forms a SQUID superconducting loop.

2. The structure according to claim 1, characterized in that: The tilt angle θ ranges from 45° to 85°.

3. The structure according to claim 1, characterized in that: The SQUID superconducting loop has a geometric height of 5–20 nm and a geometric width of 10–100 nm.

4. The structure according to claim 1, characterized in that: The plane containing the 3D nanobridge is perpendicular to the plane containing the SQUID superconducting loop.

5. The structure according to claim 1, characterized in that: The first superconducting thin film layer, the second superconducting thin film layer, and the third superconducting thin film layer are made of the same material.

6. The structure according to claim 1, characterized in that: The widths of the first superconducting thin film layer, the insulating layer, and the second superconducting thin film layer decrease sequentially.

7. The structure according to claim 1, characterized in that: The thickness of the first superconducting thin film layer is 5–300 nm, the thickness of the second superconducting thin film layer is 5–300 nm, and the thickness of the third superconducting thin film layer is 5–30 nm.

8. The structure according to any one of claims 1 to 7, characterized in that: The insulating layer is made of one of SiO2, MgO, AlN or Al2O3, and the thickness of the insulating layer is 5 to 20 nm.

9. A method for fabricating a SQUID superconducting loop area structure, characterized in that, Includes the following steps: A silicon substrate is provided, a silicon dioxide thin film is formed on the silicon substrate, and a stacked structure is formed on the silicon dioxide thin film, the stacked structure comprising a first superconducting thin film layer, an insulating layer and a second superconducting thin film layer deposited sequentially. The stacked structure is exposed and subjected to reactive ion beam etching, so that the shape of the stacked structure is an isosceles trapezoid, and both sidewalls of the stacked structure form slopes with an inclination angle θ with the silicon substrate. A third superconducting thin film layer is deposited again on the slope and the silicon substrate near the stacked structure. Electron beam lithography is performed on the third superconducting thin film layer. By controlling the different exposure doses, 3D nanobridges are formed on the slope of the insulating layer. The 3D nanobridges are symmetrically arranged on both sides of the slope of the stacked structure.

10. The preparation method according to claim 9, characterized in that: The materials of the first superconducting thin film layer, the second superconducting thin film layer, and the third superconducting thin film layer include one of Nb, NbN, Al, and Pb.

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