Mechanical resonator, methods for its manufacture and its use

Abstract

The present invention relates to a mechanical resonator for acceleration and position sensors, which has a compact design and simultaneously exhibits a low natural frequency and a high quality factor. The invention also relates to a method for manufacturing this mechanical resonator. The mechanical resonator is used, for example, in gravitational wave detectors, atom interferometers, and EUV lithography systems. The position of the test specimen of the mechanical resonator can then subsequently be read out interferometrically using an optical system.

Description

[0001] The present invention relates to a mechanical resonator for acceleration and position sensors that has a compact design and simultaneously exhibits a low natural frequency with a high quality factor. The invention also relates to a method for manufacturing this mechanical resonator. The mechanical resonator is used, for example, in gravitational wave detectors, atom interferometers, and EUV lithography systems. The position of the test specimen of the mechanical resonator can then be read out later interferometrically using an optical system.

[0002] Acceleration and position sensors are used in many technical measurement systems. In some cases, space in the measurement systems is limited, so the resonator, i.e., the part of the sensor that detects external mechanical disturbances, must have a compact geometry. Current technology is that compact resonators have a low quality factor, i.e., low sensitivity, and / or a high natural frequency, meaning they can only detect the low-frequency component of external mechanical disturbances with poor sensitivity.

[0003] There is currently no resonator that is compact, has a low natural frequency and a high quality factor.

[0004] Gravitational wave detectors require the precise measurement of low-frequency seismic disturbances. Commercial seismometers such as the Sercel L-4C or Geotech GS-13 are currently used for this purpose (S. Cooper, C. Collins, L. Prokhorov et al., 2022: Interferometric sensing of a commercial geophone. Class. Quantum Grav. 39(7), https: / / doi.org / 10.1088 / 1361-6382 / ac595c). These seismometers have a natural frequency of 1 Hz and weigh several kilograms, making them correspondingly large. The disadvantage is that, due to limited space, measurements cannot be taken directly near the large interferometer mirrors.

[0005] Atom interferometers utilize the quantum mechanical wave properties of atoms to make precise measurements of fundamental natural constants, such as the gravitational constant or the fine-structure constant. Current atom interferometers use compact monolithic glass resonators. These are closest to the present invention (LL R ICHARDSON , A. R AJAGOPALAN , H.A LBERS et al., 2020: Optomechanical resonator-enhanced atom interferometry. Commun. Phys. 3(208), https: / / doi.org / 10.1038 / s42005-020-00473-4). The disadvantage is that their natural frequency is relatively high (f0 = 678 Hz), because the geometry of the resonators leads to high stiffness, and the quality factor is low (Q = 630).

[0006] Active vibration isolation systems are used in EUV lithography systems. They work with actuators and sensors to measure, control, and isolate incoming vibrations. Currently, capacitive and piezoelectric sensors are used, but these are particularly sensitive to higher frequencies (S. Spanjer, W. Hakvoort, 2021: Optimal Active Vibration Isolation Systems for Multiple Noise Sources. European Control Conference 2021. In Proc. IEEE, p. 2473, https: / / doi.org / 10.23919 / ECC54610.2021.9654949). The disadvantage is that ambient electromagnetic fields and cables associated with the sensor can distort the resonator readings, and that low-frequency seismic disturbances cannot be completely isolated due to the sensors' insensitivity in this frequency range.

[0007] Based on this, it was the object of the present invention to provide a mechanical resonator which overcomes the described disadvantages of the prior art and has a compact design and a low natural frequency with a high quality factor.

[0008] This object is achieved by the method for producing a mechanical resonator for acceleration and position sensors having the features of claim 1, the mechanical resonator having the features of claim 13, and the use according to claim 15. The further dependent claims list advantageous developments.

[0009] According to the invention, a method for producing a mechanical resonator for acceleration and position sensors is provided, comprising the following steps: a) providing a first substrate and two cover substrates, each containing or consisting of a material selected from the group consisting of glass, glass ceramic, sapphire, silicon, silicon carbide or combinations thereof, b) polishing the substrate and the cover substrates by means of chemical mechanical polishing (CMP), pitch polishing or ion beam polishing, whereby the first substrate is polished on both sides and the cover substrates are polished on one side, i.e. the respective contact surfaces of the substrates, c) producing a test specimen and a frame from the first substrate by machining or laser processing, d) Coating at least part of the test specimen with a reflective coating by means of physical vapor deposition (PVD) and / or atomic layer deposition (ALD), e) creating a bending beam geometry on the cover substrates by means of laser ablation and / or chemical etching, whereby in the areas outside the bending beam geometry on the two cover substrates, material is removed in some areas and a trench enclosing the bending beam geometry is created, f) creating holes in the cover substrates by machining or laser processing for the relative alignment of the substrates to each other, g) connecting the frame and the test specimen to the bending beams of the cover substrates by means of wafer bonding to produce a monolithic resonator, whereby the frame and the test specimen are first connected to the first cover substrate and then to the second cover substrate, h) filling the holes in the substrates with a chemically soluble putty and then curing the putty, i) Re-thinning of the cover substrates by grinding and subsequent polishing, j) removal of the putty by dissolving it with an acid, an alkali or a solvent, k) detaching the material of the cover substrates (5) from the resonator (8) which is not part of the bending beams and l) Cleaning the resonator.

[0010] The present invention thus provides a method for manufacturing a mechanical, monolithic resonator having a compact design, a low natural frequency and a high quality factor.

[0011] The concept of the present invention is based on the production of a monolithic component with very long, thin bending beams with high specific stiffness. These bending beams enable a low natural frequency while still maintaining a compact design. The quality factor is decisively influenced by the choice of material and the joining method.

[0012] The material used is selected from the group consisting of glass, glass-ceramic, sapphire, silicon, silicon carbide, or combinations thereof, which exhibit high specific stiffness and elastic deformation. Furthermore, these materials can be achieved with a very low surface roughness (square roughness). This ensures maximum reduction of internal mechanical losses during movement of the cantilever beams.

[0013] Since any joint consisting of intermediate layers, such as adhesive or solder, leads to mechanical losses when the bending beams move, the inventive solution is based on choosing a monolithic design. This is achieved through wafer bonding. Wafer bonding is a process from semiconductor technology, which is used here to create permanently stable, interlayer-free joints between silica glass surfaces. In this process, the surfaces are cleaned, chemically and / or physically activated, then brought into contact, and finally subjected to a heat treatment. The result is a covalent bond between the surfaces, so that the original two components can now be considered as one.

[0014] It is preferred that the glass is a silicate glass, in particular a quartz glass.

[0015] Preferably, the polishing in step b) is carried out to a surface roughness, measured as Sq roughness on a measuring area of ​​10 × 10 µm 2 , of a maximum of 1 nm, preferably of a maximum of 0.5 nm and especially of 0.1 to 0.3 nm. The Sq roughness is the root mean square over the area.

[0016] A preferred embodiment of the method according to the invention provides that the coating in step d) is carried out with a material selected from the group consisting of gold, silver, and aluminum. The layer thickness of the coating is preferably in the range of 50 to 500 nm, particularly preferably from 150 to 200 nm. Alternatively, it is also possible for the coating to be a dielectric layer made of silicon, silicon dioxide, and / or tantalum pentoxide, wherein the layer thickness is preferably in the range of 0.2 to 5 µm, particularly preferably from 1 to 2 µm.

[0017] The bending beam preferably has a thickness in the range of 40 to 300 µm, particularly preferably 80 to 100 µm. It is preferred that the trench surrounding the bending beam geometry preferably has a depth of d+20 to d+150 µm, preferably d+30 to d+70 µm, measured perpendicular to the substrate surface, where d is the thickness of the bending beam.

[0018] It is further preferred that in step e), the material removal is from 5 to 40 µm of the material. This refers to the material removal rate perpendicular to the substrate surface.

[0019] It is preferred that the wafer bonding in step g) involves plasma-activated bonding (PAB) followed by a temperature treatment at a temperature in the range of 80 °C to 450 °C, preferably 200 to 250 °C.

[0020] Preferably, the putty in step h) is selected from the group consisting of epoxy resins, ketone formaldehyde resins, polythiols or combinations thereof.

[0021] The re-thinning in step i) is completed with a polishing step so that a surface roughness, measured as Sq-roughness on a measuring area of ​​10 × 10 µm 2 , of a maximum of 2 nm, preferably of a maximum of 1 nm and especially of 0.3 to 0.7 nm.

[0022] It is further preferred that the dissolution of the putty in step j) is carried out by means of an acid, an alkali or a solvent selected from the group consisting of acetone, toluene, methylene chloride or mixtures thereof.

[0023] A preferred embodiment provides that for cleaning the resonator in step k) at least one of the following steps is carried out: • Storage in or manual cleaning with an acid, an alkali or a solvent selected from the group consisting of acetone, toluene, methylene chloride or mixtures thereof. • Megasonic treatment of the resonator with deionized water.

[0024] According to the invention, a mechanical resonator for acceleration and position sensors is further provided, which includes a test piece made of a first substrate and surrounded by a frame. The frame and the test piece are monolithically connected at the front and rear sides to a cover substrate in the form of bending beams. This resonator can be manufactured using the method described above.

[0025] Preferably, the resonator has at least one of the following properties: • a test specimen mass in the range of 1 to 20 g, preferably 2 to 5 g, • a natural frequency of up to 20 Hz, preferably from 0.1 to 15 Hz, • a quality factor of at least 10,000, preferably from 100,000 to 1,000,000.

[0026] The mechanical resonator according to one of claims 13 to 15 is used in gravitational wave detectors, atom interferometers, EUV lithography systems, X-ray lithography systems or navigation devices.

[0027] The subject matter of the invention will be explained in more detail with reference to the following example and the figures, without wishing to restrict it to the specific embodiments shown here. Fig. 1 shows in a perspective view the frame of a resonator according to the invention Fig. 2 shows a perspective view of the test specimen of a resonator according to the invention Fig. 3 shows a plan view of a cover substrate of a resonator according to the invention Fig. 4 shows a perspective view of a resonator according to the invention

[0028] In Fig. Figure 1 shows a frame 1 created from the first substrate by machining. On the lower, outer side is a flat surface 2, which serves for the installation of the final mechanical resonator in an optical system. Due to the fact that a round substrate was used as the starting semi-finished product, only the inner area 3 had to be machined, except for the flat surface 2. The shape of the inner surface exhibits threefold rotational symmetry. The protruding areas on the surfaces are contact surfaces with the bending beams.

[0029] In Fig. Figure 2 shows a test specimen 4, which was also created from the first substrate by machining. It also exhibits threefold rotational symmetry. The protruding areas on the surfaces are contact surfaces with the bending beams. A reflective coating is applied to the center of one surface in a later step.

[0030] In Fig. Figure 3 shows a cover substrate 5, in one of whose surfaces material was removed around the bending beam geometry using laser ablation, and a hole was created using machining. The bending beam geometry here corresponds to the three long, narrow, curved surfaces 6, 6', 6", the ends of which are each slightly wider and will serve as the contact surfaces with the frame and the test specimen. Hole 7 is located between the bending beams. It is important to note that the second cover substrate is prepared in a corresponding mirror image.

[0031] In Fig.Figure 4 shows the final mechanical resonator 8. The element has threefold rotational symmetry (except for the flat surface on the underside on the outside of the frame). The front and back of the resonator each have three cantilever beams 9, 9', 9", which monolithically connect the frame and the test piece. The reflective coating is applied to the center of the front of the resonator. The present design allows the test piece to oscillate at a very low natural frequency despite its small mass; due to the rotational symmetry, the test piece moves only along the optical axis. The choice of material, the surface finish of the cantilever beams, and the monolithic nature of the resonator enable a very high quality factor. Example

[0032] The starting semi-finished products are a first substrate measuring 50 mm in diameter x 6.35 mm in thickness, and cover substrates measuring 50 mm in diameter x 1 mm in thickness. These substrates are pre-polished with a good flatness of better than 400 nm (peak-to-valley, test area 80% of the area). The substrate material is very pure fused silica (Corning 7980 0F). All substrates are then subjected to a final polishing with CMP on both sides.

[0033] From the polished round disc (first substrate), both the frame and the test specimen are manufactured using ultrasonic milling. The resulting test specimen has a mass of 3 g, the frame a mass of 12 g, and the entire resonator a mass of 15 g. The resonator has a natural frequency of 15 Hz.

[0034] A 20 nm thick titanium adhesion layer is deposited on the test specimen using a mechanical mask and the magnetron sputtering process, followed by a 200 nm thick gold layer as a reflective coating.

[0035] This is followed by the creation of a bending beam geometry on the cover substrates using laser ablation. In the areas outside the bending beam geometry, a 10 µm material removal is performed on the two cover substrates, and a 150 µm deep trench is created enclosing the bending beam geometry. A laser is then used to create a marking for the hole. The holes in the cover substrates are created using ultrasonic milling, with the diameter of the holes being 2.0 mm.

[0036] The frame and test specimen are bonded to the first cover substrates using plasma-activated bonding. This is based on the following steps: • chemical activation of the surfaces with diluted ammonia water and hydrogen peroxide, • DI water treatment with megasound, • Spin drying, • Plasma activation of the surfaces with N2 and O2 plasma, • DI water treatment with megasound, • Spin drying • Contacting in a device that uses mechanical stops for alignment, • Temperature treatment (250°C for 8h in vacuum, no external mechanical pressure).

[0037] The back of the resulting element is connected to the second cover substrate using the same steps, but a test pin (diameter 1.95 mm) is also used for alignment in the device.

[0038] Next, the holes in the substrates are filled with a soluble putty, which is initially liquid but then cured by UV radiation. The putty hardens sufficiently to stabilize the component for the re-thinning process.

[0039] The re-thinning process involves conventional grinding and polishing, followed by a final polishing step using CMP. This process thins the cover substrates to such an extent that only 100 µm of material remains. The cement is then removed by soaking the component in acetone. Due to the previously prepared trenches in the cover substrates, the material of the cover substrates that is not part of the cantilever beams is now removed from the resonators. Only the cantilever beams (now polished on both sides) remain, with a thickness of 100 µm.

[0040] A final cleaning is done by wiping with a cotton swab containing acetone and a local megasonic treatment with DI water.

Claims

[1] Method for producing a mechanical resonator (8) for acceleration and position sensors, comprising the following steps: a) providing a first substrate and two cover substrates (5), each containing or consisting of a material selected from the group consisting of glass, glass ceramic, sapphire, silicon, silicon carbide or combinations thereof, b) polishing the substrate and the cover substrates (5) by means of chemical-mechanical polishing, pitch polishing or ion beam polishing, c) producing a test specimen (4) and a frame (1) from the first substrate by means of machining or laser processing, d) coating at least part of the test specimen (4) with a reflective coating by means of physical vapor deposition (PVD) and / or atomic layer deposition (ALD), e) producing a bending beam geometry (6, 6', 6'') on the cover substrates (5) by means of laser ablation and / or chemical etching, wherein in the areas outside the bending beam geometry (6, 6', 6") on the two cover substrates (5) a material removal takes place in some areas and a trench enclosing the bending beam geometry (6, 6', 6") is produced, f) creating holes in the cover substrates (5) by machining or laser processing for the relative alignment of the substrates to each other, g) connecting the frame (1) and the test specimen (4) to the bending beams (9, 9', 9") of the cover substrates (5) by means of wafer bonding to produce a monolithic resonator (8), wherein first the frame (1) and the test specimen (4) are connected to the first cover substrate and then to the second cover substrate, h) filling the holes in the substrates with a chemically soluble putty and then curing the putty, i) Re-thinning of the cover substrates (5) by grinding or lapping and subsequent polishing, j) removal of the putty by dissolving it with an acid, an alkali or a solvent, k) detaching the material of the cover substrates (5) from the resonator (8) which is not part of the bending beams (9, 9', 9"), and I) Cleaning the resonator (8). [2] Method according to claim 1, characterized by that the glass is a silicate glass, in particular a quartz glass. [3] Method according to claim 1 or 2, characterized by that the polishing in step b) is carried out to a surface roughness, measured as Sq-roughness on a measuring area of ​​10 × 10 µm 2 , of a maximum of 1 nm, preferably of a maximum of 0.5 nm and especially of 0.1 to 0.3 nm. [4] Method according to one of claims 1 to 3, characterized by that machining is carried out by milling, in particular by ultrasonic-assisted milling. [5] Method according to one of claims 1 to 4, characterized by that the coating in step d) is carried out with a material selected from the group consisting of gold, silver and aluminum, wherein the layer thickness is preferably in the range from 50 to 500 nm, particularly preferably from 150 to 200 nm, or that the coating in step d) is a dielectric layer made of silicon, silicon dioxide and / or tantalum pentoxide, wherein the layer thickness is preferably in the range from 0.2 to 5 µm, particularly preferably from 1 to 2 µm. [6] Method according to one of claims 1 to 5, characterized by that the bending beams (9, 9', 9") have a thickness in the range from 40 to 300 µm, preferably from 80 to 100 µm, wherein the trench enclosing the bending beam geometry preferably has a depth of d+20 to d+150 µm, preferably from d+30 to d+70 µm, measured perpendicular to the substrate surface, where d is the thickness of the bending beams (9, 9', 9"). [7] Method according to one of claims 1 to 6, characterized by that in step e) the material removal is from 5 to 40 µm of the material. [8] Method according to one of claims 1 to 7, characterized by that during wafer bonding in step g), plasma-activated bonding (PAB) is carried out with a subsequent temperature treatment at a temperature in the range of 80 °C to 450 °C, preferably 200 to 250 °C. [9] Method according to one of claims 1 to 8, characterized by that the putty in step h) is selected from the group consisting of epoxy resins, ketone formaldehyde resins, polythiols or combinations thereof. [10] Method according to one of claims 1 to 9, characterized by that the re-thinning in step i) is completed with a polishing step so that a surface roughness, measured as Sq-roughness on a measuring area of ​​10 × 10 µm 2, of a maximum of 2 nm, preferably of a maximum of 1 nm and in particular of 0.3 to 0.7 nm. [11] Method according to one of claims 1 to 10, characterized by that the dissolution of the putty in step j) is carried out by means of an acid, an alkali or a solvent selected from the group consisting of acetone, toluene, methylene chloride or mixtures thereof. [12] Method according to one of claims 1 to 11, characterized by that for cleaning the resonator (8) in step I) at least one of the following steps is carried out: • Storage in or manual cleaning with an acid, an alkali or a solvent selected from the group consisting of acetone, toluene, methylene chloride or mixtures thereof, • Megasonic treatment of the resonator (8) with deionized water. [13] Mechanical resonator (8) for acceleration and position sensors comprising a test body (4) made of a first substrate and surrounded by a frame (1), wherein the frame (1) and the test body (4) are monolithically connected at the front and rear to a cover substrate (5) in the form of bending beams (9, 9', 9"), and producible by the method according to one of claims 1 to 12. [14] Mechanical resonator (8) according to claim 13, characterized by that the resonator (8) has at least one of the following properties: • a test specimen mass in the range of 1 to 20 g, preferably 2 to 5 g, • a natural frequency of up to 20 Hz, preferably from 0.1 to 15 Hz, • a quality factor of at least 10,000, preferably from 100,000 to 1,000,000. [15] Use of the mechanical resonator (8) according to one of claims 13 or 14 in gravitational wave detectors, atom interferometers, EUV lithography systems, X-ray lithography systems or navigation devices.

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