MEMS resonator structure and preparation method thereof

Abstract

According to the MEMS resonator structure and the preparation method thereof provided by the invention, the resonator punch-through anchor point and the resonator sprite pattern are formed through a one-time photoetching process, so that registration deviation caused by multiple times of photoetching is avoided, the resonator punch-through anchor point can be ensured to be accurately positioned at a theoretical fixed node of the resonator, the consistency of the resonator is improved, and the production cost is reduced. The quality factor of the device is easier to approach the theoretical maximum value; the connection with the harmonic oscillator is realized through the first connecting part, and the characteristic size of the cross section of the resonator punch-through anchor point is set to be far smaller than the characteristic size of the harmonic oscillator, so that the influence of the resonator punch-through anchor point on the Q value of the device is greatly reduced; the resonator punch-through anchor points are formed in a heavily doped polycrystalline silicon filling mode, firm mechanical anchoring between the harmonic oscillator and the substrate is achieved, the mechanical stability and impact resistance of the MEMS resonator structure are guaranteed, meanwhile, a reliable electrical connection path is formed between the harmonic oscillator and a lower layer circuit, and the reliability of a device is improved.

Description

Technical Field

[0001] This invention relates to the field of sensor technology, and in particular to a MEMS resonator structure and its fabrication method. Background Technology

[0002] The high-frequency signals generated by clock oscillators can be used as clock signals in electronic systems. They are core components in electronic systems and are widely used in communications, computing, and consumer electronics, with a market size reaching tens of billions of yuan. Currently, mainstream clock oscillators are made of quartz crystal material, which has a relatively large structural size and is difficult to be compatible with integrated circuit processes. However, with the rapid development of mobile communications, wearable devices, and other fields, electronic systems are placing higher demands on the miniaturization and integration of devices.

[0003] Microelectromechanical systems (MEMS) technology, as a high-tech field integrating electronics, mechanics, optics, and biotechnology, developed on the basis of microelectronic IC technology, has provided a new direction for the development of oscillators. MEMS silicon-based oscillators are a new generation of oscillators made of silicon using MEMS technology. They have advantages such as small size, high frequency (up to GHz level), easy integration with integrated circuits, and tolerance to high-impact environments, and are considered a highly promising solution to replace quartz oscillators.

[0004] A typical MEMS oscillator usually consists of a resonator and interface circuitry. The resonator, as the core component, directly determines the overall performance of the MEMS oscillator. To achieve high sensitivity and low phase noise, it is necessary to minimize the resonator's energy loss, especially under high-vacuum packaging conditions. The resonator's vibration waves are emitted to the substrate through the anchor point, causing vibrational energy loss, making anchor loss the main energy loss of the resonator. Therefore, suppressing anchor loss is a crucial problem that must be solved in MEMS resonator development.

[0005] Theoretical studies show that placing the anchor point at the node where the resonator's vibration is zero can reduce the anchor point loss to near zero; for example, the midpoint of a square Lamé mode resonator is the theoretical point of zero vibration. However, in actual manufacturing, process tolerances are unavoidable, making anchor point losses often difficult to ignore. Specifically, this manifests in the following aspects:

[0006] 1) Although the node where the resonator vibrates to 0 is theoretically a point or a line, the cross-sectional dimensions of the actual anchor point structure are not 0, which will introduce a certain anchor point loss.

[0007] 2) The anchor point and the resonator are formed by step-by-step photolithography, and there is an overlay error between the two photolithography steps, which will cause the anchor point loss to be significantly greater than the theoretical value.

[0008] 3) Reducing the bonding area of ​​the resonator center anchor point will significantly reduce the bonding success rate, and the bonding process will also make it difficult to ensure the symmetry of the bonding surface relative to the structure, which will also introduce additional anchor point losses.

[0009] 4) In order to ensure the quality of the central anchor point, the resonator is covered with sacrificial layer material. In order to release the resonator, the sacrificial layer material under the resonator must be completely etched away. The etching process is time-consuming and the supporting structure of the electrode anchor point and frame area must be protected from corrosion, making the process difficult.

[0010] Therefore, it is necessary to propose a novel method for fabricating the center anchor point structure of a MEMS resonator, which can ensure that the anchor point is located at a node with zero vibration, while also overcoming the difficulties in alignment, bonding, and release in existing fabrication processes, thereby suppressing anchor point loss and improving the manufacturing yield of MEMS resonators. Summary of the Invention

[0011] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a MEMS resonator structure and its fabrication method, which solves the problems of process tolerance, anchor point loss and high fabrication difficulty in the actual manufacturing process of MEMS resonators in the prior art.

[0012] To achieve the above and other related objectives, the present invention provides a method for fabricating a MEMS resonator structure, the method comprising the following steps:

[0013] A first substrate and a second substrate are provided. The first substrate includes a front side and a back side disposed opposite to each other. A first oxide layer is formed on the front side of the first substrate. The second substrate is bonded to the first oxide layer and the second substrate is thinned so that the thickness of the second substrate meets the requirements of the MEMS resonator.

[0014] A composite mask layer is formed on the surface of the second substrate. The composite mask layer includes a stacked first mask layer, a second mask layer, and a third mask layer. The composite mask layer is patterned to form a resonator pattern, a resonator through-anchor pattern, a fixed through-anchor pattern, and an airtight through-anchor pattern.

[0015] A first photoresist layer is formed on the resonator pattern, and the composite mask layer is etched to form a second trench at the resonator through-anchor pattern, a third trench at the fixed through-anchor pattern, and a fourth trench at the hermetically sealed through-anchor pattern; and heavily doped polysilicon is deposited in the second trench, the third trench, and the fourth trench respectively to form the resonator through-anchor, the fixed through-anchor, and the hermetically sealed through-anchor.

[0016] A third photoresist layer is formed on the resonator through-anchor point, the fixed through-anchor point, and the hermetically sealed through-anchor point. A first etching process is performed on the resonator pattern until the second substrate is etched through to form a fifth trench. The fifth trench divides the second substrate into a resonator, an electrode, and a closed surrounding frame. Finally, a wet etching process is used to remove the first mask layer, the second mask layer, the third mask layer, and the first oxide layer not surrounded by the fixed through-anchor point and the hermetically sealed through-anchor point.

[0017] Optionally, before bonding the second substrate to the first oxide layer, the method further includes forming a plurality of first trenches on the front side of the first substrate, and growing a dielectric material resistant to hydrofluoric acid corrosion in the first trenches using a low-pressure chemical vapor deposition process to form an insulating structure, wherein the thickness of the insulating structure is less than or equal to the thickness of the first substrate.

[0018] Optionally, after forming the resonator, the electrode, and the closed surrounding frame, the method further includes a step of thinning the back side of the first substrate, and the insulating structure is exposed after thinning.

[0019] Optionally, the steps of forming the second trench, the third trench, and the fourth trench include: forming a first photoresist layer on the resonator pattern; performing a first etching process on the composite mask layer at the resonator through-anchor pattern, the fixed through-anchor pattern, and the hermetically sealed through-anchor pattern until the second substrate is etched through; then performing a second etching process at the resonator through-anchor pattern, the fixed through-anchor pattern, and the hermetically sealed through-anchor pattern until the first oxide layer is etched through; and then performing the first etching process again at the resonator through-anchor pattern, the fixed through-anchor pattern, and the hermetically sealed through-anchor pattern to etch the first substrate, wherein the etching endpoint at the location not covered by the first photoresist layer stops at the first mask layer.

[0020] Optionally, the first etching process is a deep reactive ion etching process, and the second etching process is a reactive ion etching process.

[0021] Optionally, both the first and third mask layers are silicon oxide layers, the second mask layer is a polysilicon layer, and the thickness D1 of the first mask layer should satisfy: The thickness D3 of the third mask layer should satisfy: The thickness D2 of the second mask layer should satisfy: In the first etching process, the etching selectivity ratio of silicon to the first mask layer and the third mask layer is m:n, the thickness of the second substrate is H, and the etching depth of the first substrate is h. In the second etching process, the etching selectivity ratio of the first oxide layer to the second mask layer is v:u, and the thickness of the first oxide layer is t.

[0022] Optionally, the heavily doped polysilicon has the same doping type as the first substrate and the second substrate, and the doping concentration of the heavily doped polysilicon is greater than or equal to 10. 19 / cm 3 .

[0023] Optionally, a cover wafer is further disposed on the second substrate and the cover wafer is connected to the closed surrounding frame, and a third vacuum gap is formed between the cover wafer and the second substrate.

[0024] Optionally, the third gap is filled with a non-volatile getter.

[0025] The present invention also provides a MEMS resonator structure, the MEMS resonator structure comprising:

[0026] A first substrate has a front side and a back side disposed opposite to each other. A plurality of first deep trenches are formed on the front side of the first substrate, and the first deep trenches are filled with a dielectric material to form an insulating structure.

[0027] A resonator, wherein a resonator through-anchor point is formed in the resonator and fixedly connected to the first substrate through the resonator through-anchor point, and a first gap is formed between the resonator and the first substrate;

[0028] The electrodes are symmetrically distributed on both sides of the resonator and fixed through-anchor points are formed in the electrodes and fixedly connected to the first substrate through the fixed through-anchor points. A first oxide layer is filled between the electrodes and the first substrate.

[0029] A closed surrounding frame is provided, which is distributed on both sides of the electrode. An airtight through-hole anchor is formed in the closed surrounding frame and fixedly connected to the first substrate through the airtight through-hole anchor. The first oxide layer is filled between the closed surrounding frame and the first substrate.

[0030] Optionally, the center of the resonator through-anchor point coincides with the center of the resonator. The resonator through-anchor point includes a first connecting part and a first end part, wherein the first connecting part is connected to the resonator, the first end part is located above the resonator and forms a second gap with the resonator, and the first connecting part and the first end part form a T-shaped structure.

[0031] Optionally, the cross-sectional feature size of the first connection portion is less than or equal to one-tenth of the feature size of the resonator.

[0032] Optionally, the fixed through-anchor points are distributed in a ring on one side of the electrode. The fixed through-anchor points include a second connecting part and a second end, wherein the second connecting part is fixedly connected to the electrode, the second end is located above the electrode and forms a second gap with the electrode, and the second connecting part and the second end form a T-shaped structure.

[0033] Optionally, the airtight through anchor points are distributed in a ring on one side of the closed surrounding frame. Each airtight through anchor point includes a third connecting part and a third end part. The third connecting part is fixedly connected to the closed surrounding frame, and the third end part is located above the closed surrounding frame and forms a second gap with the closed surrounding frame. The third connecting part and the third end part form a T-shaped structure.

[0034] Optionally, the MEMS resonator structure further includes a cover wafer, which is fixedly connected to the closed surrounding frame. The cover wafer, the resonator, and the electrode form a third vacuum gap, which is filled with a non-volatile getter.

[0035] Optionally, the materials forming the resonator through-hole anchor, the fixed through-hole anchor, and the hermetically sealed through-hole anchor are all heavily doped polycrystalline silicon. The doping type of the heavily doped polycrystalline silicon is the same as that of the first substrate, the resonator, the electrode, and the closed surrounding frame, and the doping concentration of the heavily doped polycrystalline silicon is greater than or equal to 10. 19 / cm 3 .

[0036] As described above, the MEMS resonator structure and its fabrication method of the present invention have the following beneficial effects: The resonator through-hole anchor and the resonator pattern are formed through a single photolithography process, fundamentally avoiding the registration deviation introduced by multiple photolithography steps. This ensures that the resonator through-hole anchor can be accurately located at the theoretical stationary node of the resonator, improving the consistency of the resonator and making the device's quality factor (Q value) more easily approach the theoretical maximum value. Structurally, the resonator through-hole anchor is connected to the resonator through a first connecting part, and the cross-sectional characteristic size of the resonator through-hole anchor is much smaller than the characteristic size of the resonator, significantly reducing the influence of the resonator through-hole anchor on the device's Q value. The use of heavily doped polycrystalline silicon filling... The formed through-anchor points not only achieve robust mechanical anchoring between the resonator and the substrate, ensuring the mechanical stability and shock resistance of the MEMS resonator structure, but also form a reliable electrical connection path between the resonator and the lower circuitry, improving the reliability of the device. The fixed and hermetically sealed through-anchor points can act as corrosion barrier layers for the first oxide layer under the electrodes and the closed surrounding frame, effectively protecting the first oxide layer directly below it from corrosion by the etchant. This ensures that the oxide layer in key support areas such as the electrode anchor points and the closed surrounding frame is preserved, maintaining the integrity of the structure and solving the over-corrosion problem caused by the long release corrosion time in large-size MEMS resonator structures. Attached Figure Description

[0037] Figure 1 The diagram shows a process flow diagram of the fabrication method of the MEMS resonator structure of the present invention.

[0038] Figure 2 The diagram shows a cross-sectional view of the first substrate of the MEMS resonator of this invention.

[0039] Figure 3 The diagram shown is a top view of the MEMS resonator of this invention after the insulation structure has been formed.

[0040] Figure 4 Displayed as Figure 3 A top view of the cross-sectional structure along AA′.

[0041] Figure 5 The diagram shows a cross-sectional structure of the first and second substrates bonded together in the MEMS resonator of this invention.

[0042] Figure 6 The diagram shows a cross-sectional structure of the MEMS resonator of this invention after the formation of the composite mask layer.

[0043] Figure 7 The diagram shows a cross-sectional structure of the composite mask layer after patterning in the MEMS resonator of this invention.

[0044] Figure 8 The diagram shows a cross-sectional structure of the MEMS resonator of the present invention after the formation of the first photoresist layer.

[0045] Figures 9 to 11 The diagram shows a cross-sectional structure of the MEMS resonator of the present invention during the formation of the second to fourth trenches.

[0046] Figure 12 The diagram shows a cross-sectional structure of the MEMS resonator of the present invention after the first photoresist layer has been removed.

[0047] Figure 13 The diagram shows a cross-sectional structure of the MEMS resonator of this invention after being filled with heavily doped polycrystalline silicon.

[0048] Figure 14 The diagram shows a cross-sectional structure of the MEMS resonator of this invention after the formation of the second photoresist layer.

[0049] Figure 15 The diagram shows a cross-sectional view of the MEMS resonator of this invention after the fifth trench has been formed.

[0050] Figure 16 The diagram shows a cross-sectional structure of the MEMS resonator after wet etching.

[0051] Figure 17 The diagram shows a cross-sectional structure of the first substrate after thinning in the MEMS resonator of this invention.

[0052] Figure 18 The diagram shows a cross-sectional view of the MEMS resonator after the bonding cover plate wafer is assembled.

[0053] Component designation explanation

[0054] 101. First substrate; 102. First oxide layer; 103. Insulating structure; 1031. Low-stress silicon nitride layer; 1032. Low-stress polysilicon layer; 104. Second oxide layer; 105. Second substrate; 106. Composite mask layer; 1061. First mask layer; 1062. Second mask layer; 1063. Third mask layer; 107. Resonator pattern; 108. Resonator through-anchor point pattern; 109. Fixed through-anchor point pattern; 110. 111. Hermetic through-hole anchor point pattern; 112. First photoresist layer; 113. Second trench; 114. Third trench; 115. Fourth trench; 116. Heavily doped polysilicon; 117. Second photoresist layer; 118. Fifth trench; 119. Resonator through-hole anchor point; 120. Fixed through-hole anchor point; 121. Hermetic through-hole anchor point; 122. Cover wafer; 123. Resonator; 124. Electrode; 125. Surrounding frame; S1~S4, Steps. Detailed Implementation

[0055] The following specific examples 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. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0056] For ease of description, spatial relation terms such as “below,” “under,” “lower than,” “below,” “above,” and “upper” may be used herein to describe the relationship between one element or feature shown in the accompanying drawings and other elements or features. It will be understood that these spatial relation terms are intended to include directions other than those depicted in the drawings for devices in use or operation. Furthermore, when a layer is referred to as being “between” two layers, it may be the only layer between the two layers, or there may be one or more layers in between.

[0057] It should be understood that the use of terms such as "first" and "second" to define the components is merely for the purpose of distinguishing the aforementioned components. Unless otherwise stated, these terms have no special meaning and therefore should not be construed as limiting the scope of protection of this invention.

[0058] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "joining," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0059] Please see Figures 1 to 18 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.

[0060] Example 1

[0061] This embodiment provides a method for fabricating a MEMS resonator structure, such as... Figure 1 As shown, it is a schematic diagram of the process flow of this preparation method, including the following steps:

[0062] S1: Provide a first substrate 101 and a second substrate 105. The first substrate 101 includes a front side and a back side disposed opposite to each other. A first oxide layer 102 is formed on the front side of the first substrate 101. The second substrate 105 is bonded to the first oxide layer 102 and the second substrate 105 is thinned so that the thickness of the second substrate 105 meets the requirements of the MEMS resonator.

[0063] S2: A composite mask layer 106 is formed on the surface of the second substrate 105. The composite mask layer 106 includes a first mask layer 1061, a second mask layer 1062 and a third mask layer 1063 stacked together. The composite mask layer 106 is patterned to form a resonator pattern 107, a resonator through-anchor pattern 108, a fixed through-anchor pattern 109 and an airtight through-anchor pattern 110.

[0064] S3: A first photoresist layer 111 is formed on the resonator pattern 107, and the composite mask layer 106 is etched to form a second trench 112 at the resonator through-anchor pattern 108, a third trench 113 at the fixed through-anchor pattern 109, and a fourth trench 114 at the hermetically sealed through-anchor pattern 110; and heavily doped polysilicon 115 is deposited in the second trench 112, the third trench 113, and the fourth trench 114 respectively to form a resonator through-anchor 118, a fixed through-anchor 119, and a hermetically sealed through-anchor 120.

[0065] S4: A third photoresist layer is formed on the resonator through-anchor point 118, the fixed through-anchor point 119, and the hermetically sealed through-anchor point 120. A first etching process is performed on the resonator pattern 107 until the second substrate 105 is etched through to form a fifth trench 117. The fifth trench 117 divides the second substrate 105 into a resonator 122, an electrode 123, and a closed surrounding frame 124. Finally, a wet etching process is used to remove the first mask layer, the second mask layer 1062, the third mask layer, and the first oxide layer 102 that is not surrounded by the fixed through-anchor point 119 and the hermetically sealed through-anchor point 120.

[0066] The fabrication method of the aforementioned MEMS resonator structure is further described below with reference to the accompanying drawings:

[0067] In step S1, please refer to Figures 1 to 5A first substrate 101 and a second substrate 105 are provided. The first substrate 101 includes a front side and a back side disposed opposite to each other. A first oxide layer 102 is formed on the front side of the first substrate 101. The second substrate 105 is bonded to the first oxide layer 102 and the second substrate 105 is thinned so that the thickness of the second substrate 105 meets the requirements of the MEMS resonator.

[0068] Specifically, such as Figure 2 The diagram shows a cross-sectional view of the first substrate 101. The first substrate 101 has a front side and a back side disposed opposite to each other. The material of the first substrate 101 can be a silicon-based material. In addition, the material of the first substrate 101 can also include other elemental semiconductors (such as germanium) or other compound semiconductors (such as silicon carbide, gallium arsenide, indium arsenide, indium phosphide, etc.). Preferably, in this embodiment, the first substrate 101 is a single-crystal silicon substrate.

[0069] As an example, before bonding the second substrate 105 to the first oxide layer 102, the method further includes forming a plurality of first trenches on the front side of the first substrate 101, and growing a dielectric material resistant to hydrofluoric acid corrosion in the first trenches using a low-pressure chemical vapor deposition process to form an insulating structure 103, wherein the thickness of the insulating structure 103 is less than or equal to the thickness of the first substrate 101.

[0070] Specifically, such as Figure 4 As shown, a first oxide layer 102 is formed on the front side of the first substrate 101 first. The first oxide layer 102 can be a silicon oxide layer obtained by thermal oxidation growth. Compared with a silicon oxide layer deposited by chemical vapor deposition (CVD), the first oxide layer 102 obtained by thermal oxidation growth has a higher density and is less susceptible to corrosion. Based on the first oxide layer 102, the first substrate 101 is etched to form several parallel first trenches. The thickness of the insulating structure 103 is less than or equal to the thickness of the first substrate 101. Since a thinning process is subsequently performed on the back side of the first substrate 101, to save on the thinning of the first trenches, the first trenches are chosen to have a certain depth but do not penetrate the first substrate 101, such as... Figure 4 As shown, a dielectric material resistant to hydrofluoric acid corrosion of a certain thickness is grown by low-pressure chemical vapor deposition in the first trench to form an insulating structure 103. Figure 3As shown, the insulating structures 103 are all closed ring structures. Optionally, the dielectric material can be a hydrofluoric acid resistant insulating material with an outer silicon nitride layer and an inner polycrystalline silicon layer. Specifically, in this embodiment, the insulating structure 103 includes a low-stress silicon nitride layer 1031 and a low-stress polycrystalline silicon layer 1032 surrounded by the silicon nitride layer. Then, the insulating structure 103 is etched to remove the low-stress silicon nitride layer 1031 and the low-stress polycrystalline silicon layer 1032 on the surface of the first trench, so that the top of the insulating structure 103 is flush with the front side of the first substrate 101.

[0071] Specifically, such as Figure 5 As shown, a second substrate 105 is provided. The second substrate 105 includes a first surface and a second surface disposed opposite to each other. The material of the second substrate 105 can be a silicon-based material. In addition, the material of the second substrate 105 can also include other elemental semiconductors (such as germanium) or other compound semiconductors (such as silicon carbide, gallium arsenide, indium arsenide, indium phosphide, etc.). Preferably, the second substrate 105 is a single-crystal silicon substrate. Optionally, in other embodiments, a second oxide layer 104 can be formed on the first surface of the second substrate 105, and the first surface of the second substrate 105 can be bonded to the first oxide layer 102. Since both the first oxide layer 102 and the second oxide layer 104 are silicon dioxide, the bonding between the second substrate 105 and the first substrate 101 is a silicon-silicon bond, thereby avoiding the incompatibility problem between the metal intermediate bonding layer and subsequent high-temperature processes, and greatly improving the reliability of the MEMS resonator structure. Furthermore, the second surface of the second substrate 105 is thinned so that the second substrate 105, as a structural layer, can form the resonator 122, the electrode 123 and the closed surrounding frame 124 through subsequent processes, and the thickness of the formed resonator 122, the electrode 123 and the closed surrounding frame 124 can meet the requirements of the MEMS resonator structure.

[0072] In step S2, please refer to Figure 1 , Figure 6 and Figure 7 A composite mask layer 106 is formed on the surface of the second substrate 105. The composite mask layer 106 includes a first mask layer 1061, a second mask layer 1062, and a third mask layer 1063 stacked together. The composite mask layer 106 is patterned to form a resonator pattern 107, a resonator through-anchor pattern 108, a fixed through-anchor pattern 109, and an airtight through-anchor pattern 110.

[0073] Specifically, in this embodiment, such as Figure 6As shown, a low-pressure chemical vapor deposition process is performed on the surface of the second substrate 105 to form a composite mask layer 106. The composite mask layer 106 includes a stacked first mask layer 1061, a second mask layer 1062, and a third mask layer 1063. Photoresist is spin-coated onto the third mask layer 1063 to form a photoresist layer. The composite mask layer 106 is etched using the photoresist layer as a mask, as shown. Figure 7 As shown, after etching through the first mask layer 1061, the second mask layer 1062 and the third mask layer 1063, a resonator pattern 107, a resonator through-anchor pattern 108, a fixed through-anchor pattern 109 and a hermetically sealed through-anchor pattern 110 are formed in the second substrate 105, and then the photoresist layer is removed by an ashing process.

[0074] In step S3, please refer to Figure 1 , Figures 8 to 12 A first photoresist layer 111 is formed on the resonator pattern 107. The composite mask layer 106 is etched to form a second trench 112 at the resonator through-anchor pattern 108, a third trench 113 at the fixed through-anchor pattern 109, and a fourth trench 114 at the hermetically sealed through-anchor pattern 110. Heavily doped polysilicon 115 is deposited in the second trench 112, the third trench 113, and the fourth trench 114 to form resonator through-anchors 118, 119, and 120, respectively.

[0075] As an example, the steps of forming the second trench 112, the third trench 113, and the fourth trench 114 include: forming a first photoresist layer 111 on the resonator pattern 107; performing the first etching process on the composite mask layer 106 at the resonator through-anchor pattern 108, the fixed through-anchor pattern 109, and the hermetically sealed through-anchor pattern 110 until the second substrate 105 is etched through; then continuing to perform a second etching process at the resonator through-anchor pattern 108, the fixed through-anchor pattern 109, and the hermetically sealed through-anchor pattern 110 until the first oxide layer 102 is etched through; and again performing the first etching process on the resonator through-anchor pattern 108, the fixed through-anchor pattern 109, and the hermetically sealed through-anchor pattern 110 to etch the first substrate 101, with the etching endpoint at the part not covered by the first photoresist layer 111 stopping at the first mask layer.

[0076] Specifically, such as Figure 8As shown, a second photoresist layer 116 is formed on the resonator pattern 107, the second photoresist layer 116 completely covers the resonator pattern 107, exposing the resonator through-anchor pattern 108, the fixed through-anchor pattern 109, and the hermetically sealed through-anchor pattern 110; as Figure 9 As shown, the first etching process is performed on the composite mask layer 106 at the resonator through-anchor pattern 108, the fixed through-anchor pattern 109, and the hermetically sealed through-anchor pattern 110. In this embodiment, the first etching process is a deep reactive ion etching process. The third mask layer 1063 is a silicon oxide layer, and the thickness D3 of the third mask layer 1063 should satisfy: In this process, the etching selectivity ratio of silicon to the first mask layer 1061 and the third mask layer 1063 is m:n, the thickness of the second substrate 105 is H, and the second photoresist layer 116 and the third mask layer in the composite mask have a blocking effect on ions in the deep reactive ion etching process, thereby ensuring that the second substrate 105 is etched through to form a second trench 112 at the resonator through-anchor pattern 108, a third trench 113 at the fixed through-anchor pattern 109, and a fourth trench 114 at the hermetically sealed through-anchor pattern 110; as Figure 10 As shown, a second etching process continues at the second trench 112, the third trench 113, and the fourth trench 114. In this embodiment, the second etching process is a reactive ion etching process, and the second mask layer 1062 is a polysilicon layer. The thickness D2 of the second mask layer 1062 should satisfy: In this process, the etching selectivity ratio of the first oxide layer 102 to the second mask layer is v:u, and the thickness of the first oxide layer 102 is t. The second etching process will penetrate the third mask layer in the composite mask. However, due to the low etching rate of reactive ion etching on polysilicon, the second mask layer will not be penetrated when the first oxide layer 102 is etched. Figure 11 As shown, the first etching process continues at the second trench 112, the third trench 113, and the fourth trench 114. This first etching process will penetrate the second mask layer 1062, and the etching endpoint will stop on the first mask layer 1061. The thickness D1 of the first mask layer 1061 should satisfy the following: In this process, the etching selectivity ratio of silicon to the first mask layer 1061 and the third mask layer 1063 is m:n, the thickness of the second substrate 105 is H, and the etching depth of the first substrate 101 is h, so that the second trench 112, the third trench 113 and the fourth trench 114 can be embedded into the first substrate 101.

[0077] Specifically, such as Figure 12 As shown, the second photoresist layer 116 is removed by an ashing process, as... Figure 13 As shown, heavily doped polysilicon 115 is deposited in the second trench 112, the third trench 113 and the fourth trench 114 respectively to form resonator through-anchor point 118, fixed through-anchor point 119 and hermetically sealed through-anchor point 120 respectively. At this time, heavily doped polysilicon 115 is also deposited in the resonator pattern 107 and the heavily doped polysilicon 115 covers the first mask layer 1061.

[0078] In step S4, please refer to Figure 1 , Figures 14 to 16 A third photoresist layer is formed on the resonator through-anchor point 118, the fixed through-anchor point 119, and the hermetically sealed through-anchor point 120. A first etching process is performed on the resonator pattern 107 until the second substrate 105 is etched through to form a fifth trench 117. The fifth trench 117 divides the second substrate 105 into a resonator 122, an electrode 123, and a closed surrounding frame 124. Finally, a wet etching process is used to remove the first mask layer, the second mask layer 1062, the third mask layer, and the first oxide layer 102 that is not surrounded by the fixed through-anchor point 119 and the hermetically sealed through-anchor point 120.

[0079] Specifically, such as Figure 14 As shown, a third photoresist layer is formed on the resonator through-anchor point 118, the fixed through-anchor point 119, and the hermetically sealed through-anchor point 120, exposing the resonator pattern 107, as shown. Figure 15 As shown, the first etching process is then performed on the composite mask layer 106 at the resonator pattern 107. Since the third photoresist layer and the first and third mask layers in the composite mask have a blocking effect on deep reactive ion etching, self-aligned etching can be achieved. That is, in addition to etching away the heavily doped polysilicon 115 on the surface of the third mask layer, the second substrate 105 is etched through only at the resonator pattern 107 to form a fifth trench 117. The fifth trench 117 divides the second substrate 105 into three parts: the resonator 122, the electrode 123, and the closed surrounding frame 124.

[0080] Specifically, a wet etching process is used to etch the first mask layer, the second mask layer 1062, the third mask layer, and the first oxide layer 102 and the second oxide layer 104 that are not surrounded by the fixed through-anchor point 119 and the hermetically sealed through-anchor point 120, releasing the resonator 122. That is, after the remaining second mask layer and the first mask layer are completely etched, they will detach from the entire MEMS resonator structure and be removed, forming a structure as shown in the figure. Figure 16As shown, the requirements for the etchant in the wet etching process are that the etching rate of the etchant on silicon and silicon nitride materials is much lower than that on silicon oxide materials. This includes, but is not limited to, HF etchants. Furthermore, since the electrode 123 and the first oxide layer 102 and the second oxide layer 104 under the closed surrounding frame 124 are protected by the fixed through anchor point 119 and the airtight through anchor point 120 to form a closed structure, even if the wet etching process is used for a period of time, it will not have a significant impact on the MEMS resonator structure.

[0081] As an example, the doping type of the heavily doped polysilicon 115 is the same as that of the first substrate 101 and the second substrate 105, and the doping concentration of the heavily doped polysilicon 115 is greater than or equal to 10. 19 / cm 3 .

[0082] Specifically, in this embodiment, the doping type of the heavily doped polysilicon 115 is the same as that of the first substrate 101 and the second substrate 105, for example, both being N-type doped or both being P-type doped. To reduce the series resistance of the multiple resonators 122 through-anchor points in the MEMS resonator structure, the doping concentration of the heavily doped polysilicon 115 is greater than or equal to 10. 19 / cm 3 .

[0083] As an example, after forming the resonator 122, the electrode 123 and the closed surrounding frame 124, the method further includes a step of thinning the back side of the first substrate 101, and after thinning, the insulating structure 103 is exposed.

[0084] Specifically, such as Figure 17 As shown, after forming the resonator 122, the electrode 123 and the closed surrounding frame 124, the back side of the first substrate 101 is thinned to expose the insulating structure 103, thereby achieving electrical insulation between the inside and outside of the insulating structure 103.

[0085] Example 2

[0086] The difference between this embodiment and Embodiment 1 is that, as Figure 18 As shown, after forming the resonator 122, the electrode 123 and the closed surrounding frame 124, a step of bonding the cover wafer 121 to the closed surrounding frame 124 can be performed to form a vacuum third gap between the cover wafer 121 and the second substrate 105, thereby realizing the vacuum packaging of the MEMS resonator structure.

[0087] In some embodiments, the third gap is filled with a non-volatile getter, thereby maintaining a vacuum low-pressure environment in the third gap region formed after the closed surrounding frame 124 is bonded to the cover wafer 121, and enhancing the vacuum packaging effect of the MEMS resonator structure.

[0088] Example 3

[0089] This embodiment provides a MEMS resonator structure, such as Figure 17 The diagram shows a cross-sectional view of a MEMS resonator structure. The MEMS resonator structure includes: a first substrate 101, which has a front and a back side disposed opposite to each other. The front side of the first substrate 101 has a plurality of first deep trenches, and the first deep trenches are filled with a dielectric material to form an insulating structure 103; a resonator 122, which has a resonator through-anchor point 118 and is fixedly connected to the first substrate 101 through the resonator through-anchor point 118, and a first gap is formed between the resonator 122 and the first substrate 101; and electrodes 123, which are symmetrical. Fixed through-anchor points 119 are formed in the electrodes 123 on both sides of the resonator and are fixedly connected to the first substrate 101 through the fixed through-anchor points 119. A first oxide layer 102 is filled between the electrodes 123 and the first substrate 101. A closed surrounding frame 124 is distributed on both sides of the electrodes 123 and has airtight through-anchor points 120 formed in the closed surrounding frame 124 and is fixedly connected to the first substrate 101 through the airtight through-anchor points 120. The first oxide layer 102 is filled between the closed surrounding frame 124 and the first substrate 101.

[0090] Specifically, the materials of the first substrate 101, the resonator, the electrode 123, and the closed surrounding frame 124 can be silicon-based materials. Alternatively, the materials of the first substrate 101, the resonator, the electrode 123, and the closed surrounding frame 124 can also include other elemental semiconductors (such as germanium) or other compound semiconductors (such as silicon carbide, gallium arsenide, indium arsenide, indium phosphide, etc.). Preferably, the materials of the first substrate 101, the resonator, the electrode 123, and the closed surrounding frame 124 are all single-crystal silicon. Because there is less interface loss between materials of the same type, the Q value of the MEMS resonator structure can be further improved.

[0091] As an example, the center of the resonator through-anchor 118 coincides with the center of the resonator 122. The resonator through-anchor 118 includes a first connecting part and a first end part. The first connecting part is connected to the resonator 122, and the first end part is located above the resonator 122 and forms a second gap with the resonator 122. The first connecting part and the first end part form a T-shaped structure.

[0092] Specifically, such as Figure 17 As shown, the center of the resonator through-anchor point 118 coincides with the center of the resonator 122, making the resonator through-anchor point 118 as close as possible to the node where the resonator vibration is zero, thereby making the anchor point loss infinitely close to zero. The resonator through-anchor point 118 includes a first connecting part and a first end part, wherein the first connecting part is connected to the resonator 122, and the first end part is located above the resonator 122 and forms a second gap with the resonator 122. The first connecting part and the first end part form a T-shaped structure. Preferably, the cross-sectional characteristic dimension of the first connecting part is less than or equal to one-tenth of the characteristic dimension of the resonator 122, so that the resonator 122 can be anchored in the first substrate 101 through the resonator through-anchor point 118. The characteristic dimension is the dimension that determines the resonator frequency. For a Lamé resonator, the characteristic dimension is the side length; for a circular resonator, the characteristic dimension is the diameter.

[0093] Specifically, such as Figure 17 As shown, the fixed through-anchor points 119 are distributed in a ring on one side of the electrode 123. The fixed through-anchor points 119 include a second connecting part and a second end part. The second connecting part is fixedly connected to the electrode 123, and the second end part is located above the electrode 123 and forms a second gap with the electrode 123. The second connecting part and the second end part form a T-shaped structure, so that the electrode 123 can be anchored in the first substrate 101 through the fixed through-anchor points 119.

[0094] Specifically, such as Figure 17 As shown, the airtight through anchor points 120 are distributed in a ring on one side of the closed surrounding frame 124. Each airtight through anchor point 120 includes a third connecting part and a third end part. The third connecting part is fixedly connected to the closed surrounding frame 124, and the third end part is located above the closed surrounding frame 124 and forms a second gap with the closed surrounding frame 124. The third connecting part and the third end part form a T-shaped structure, thereby enabling the closed surrounding frame 124 to be anchored in the first substrate 101 through the airtight through anchor points 120.

[0095] In some embodiments, the materials forming the resonator through-hole anchor 118, the fixed through-hole anchor 119, and the hermetically sealed through-hole anchor 120 are all heavily doped polycrystalline silicon 115. The doping type of the heavily doped polycrystalline silicon 115 is the same as that of the first substrate 101, the resonator, the electrode 123, and the closed surrounding frame 124, and the doping concentration of the heavily doped polycrystalline silicon 115 is greater than or equal to 10. 19 / cm 3 This helps reduce the series resistance of multiple resonators 122 passing through the anchor points in the MEMS resonator structure.

[0096] Example 4

[0097] This embodiment provides another MEMS resonator structure, such as Figure 18 The diagram shown is a cross-sectional view of the MEMS resonator structure in this embodiment. The difference between this embodiment and embodiment 3 is that a cover wafer 121 is also bonded to the closed surrounding frame 124.

[0098] Specifically, such as Figure 18 As shown, the MEMS resonator structure also includes a cover wafer 121, which is fixedly connected to the closed surrounding frame 124. The cover wafer 121, the resonator 122, and the electrode 123 form a vacuum third gap. The third gap is also filled with a non-volatile getter, thereby maintaining the vacuum low-pressure environment in the third gap region formed after the closed surrounding frame 124 and the cover wafer 121 are bonded, and enhancing the vacuum packaging effect of the MEMS resonator structure.

[0099] In summary, the resonator through-hole anchor and resonator in this invention are formed through a single photolithography process, fundamentally avoiding the registration deviation introduced by multiple photolithography steps. This ensures that the resonator through-hole anchor is precisely located at the theoretical stationary node of the resonator, improving the consistency of the resonator and making the device's quality factor (Q value) more likely to approach the theoretical maximum value. Structurally, the resonator through-hole anchor is connected to the resonator through a first connecting part, and the cross-sectional characteristic dimension of the resonator through-hole anchor is much smaller than that of the resonator, significantly reducing the impact of the resonator through-hole anchor on the device's Q value. The resonator through-hole anchor formed by using heavily doped polycrystalline silicon filling not only achieves... The robust mechanical anchoring between the resonator and the substrate ensures the mechanical stability and shock resistance of the MEMS resonator structure. It also forms a reliable electrical connection path between the resonator and the underlying circuitry, improving device reliability. The fixed and hermetically sealed through-hole anchors act as corrosion-resistant barriers to the first oxide layer beneath the electrodes and the closed surrounding frame, effectively protecting the first oxide layer directly below from corrosion. This ensures the preservation of the oxide layer in critical support areas such as the electrode anchors and the closed surrounding frame, maintaining structural integrity and solving the over-corrosion problem caused by long release corrosion times in large-size MEMS resonator structures. Therefore, this invention effectively overcomes the various shortcomings of existing technologies and has high industrial applicability.

[0100] 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 method for fabricating a MEMS resonator structure, characterized in that, The preparation method includes the following steps: A first substrate and a second substrate are provided. The first substrate includes a front side and a back side disposed opposite to each other. A first oxide layer is formed on the front side of the first substrate. The second substrate is bonded to the first oxide layer and the second substrate is thinned so that the thickness of the second substrate meets the requirements of the MEMS resonator. A composite mask layer is formed on the surface of the second substrate. The composite mask layer includes a stacked first mask layer, a second mask layer, and a third mask layer. The composite mask layer is patterned to form a resonator pattern, a resonator through-anchor pattern, a fixed through-anchor pattern, and an airtight through-anchor pattern. A first photoresist layer is formed on the resonator pattern, and the composite mask layer is etched to form a second trench at the resonator through-anchor point pattern, a third trench at the fixed through-anchor point pattern, and a fourth trench at the airtight through-anchor point pattern. Heavily doped polysilicon is deposited in the second trench, the third trench and the fourth trench respectively to form resonator through-hole anchors, fixed through-hole anchors and hermetically sealed through-hole anchors; A third photoresist layer is formed on the resonator through-anchor point, the fixed through-anchor point, and the hermetically sealed through-anchor point. A first etching process is performed on the resonator pattern until the second substrate is etched through to form a fifth trench. The fifth trench divides the second substrate into a resonator, an electrode, and a closed surrounding frame. Finally, a wet etching process is used to remove the first mask layer, the second mask layer, the third mask layer, and the first oxide layer not surrounded by the fixed through-anchor point and the hermetically sealed through-anchor point.

2. The method for fabricating the MEMS resonator structure according to claim 1, characterized in that: Before bonding the second substrate to the first oxide layer, the method further includes forming a plurality of first trenches on the front side of the first substrate, and growing a dielectric material resistant to hydrofluoric acid corrosion in the first trenches using a low-pressure chemical vapor deposition process to form an insulating structure, wherein the thickness of the insulating structure is less than or equal to the thickness of the first substrate.

3. The method for fabricating a MEMS resonator structure according to claim 2, characterized in that, After forming the resonator, the electrode and the closed surrounding frame, the process further includes a step of thinning the back side of the first substrate, and the insulating structure is exposed after thinning.

4. The method for fabricating the MEMS resonator anchor structure according to claim 1, characterized in that: The steps of forming the second trench, the third trench, and the fourth trench include: forming a first photoresist layer on the resonator pattern; performing a first etching process on the composite mask layer at the resonator through-anchor pattern, the fixed through-anchor pattern, and the hermetically sealed through-anchor pattern until the second substrate is etched through; then continuing to perform a second etching process at the resonator through-anchor pattern, the fixed through-anchor pattern, and the hermetically sealed through-anchor pattern until the first oxide layer is etched through; and again performing the first etching process at the resonator through-anchor pattern, the fixed through-anchor pattern, and the hermetically sealed through-anchor pattern to etch the first substrate, with the etching endpoint at the location not covered by the first photoresist layer stopping at the first mask layer.

5. The method for fabricating the MEMS resonator structure according to claim 4, characterized in that: The first etching process is a deep reactive ion etching process, and the second etching process is a reactive ion etching process.

6. The method for fabricating a MEMS resonator structure according to claim 5, characterized in that, Both the first and third mask layers are silicon oxide layers, and the second mask layer is a polysilicon layer. The thickness D1 of the first mask layer should satisfy the following: The thickness D3 of the third mask layer should satisfy: The thickness D2 of the second mask layer should satisfy: In the first etching process, the etching selectivity ratio of silicon to the first mask layer and the third mask layer is m:n, the thickness of the second substrate is H, and the etching depth of the first substrate is h. In the second etching process, the etching selectivity ratio of the first oxide layer to the second mask layer is v:u, and the thickness of the first oxide layer is t.

7. The method for fabricating a MEMS resonator structure according to claim 1, characterized in that: The heavily doped polycrystalline silicon has the same doping type as the first substrate and the second substrate, and the doping concentration of the heavily doped polycrystalline silicon is greater than or equal to 10. 19 / cm 3 .

8. The method for fabricating a MEMS resonator structure according to claim 1, characterized in that: A cover plate wafer is also disposed on the second substrate and the cover plate wafer is connected to the closed surrounding frame, and a vacuum third gap is formed between the cover plate wafer and the second substrate.

9. The method for fabricating a MEMS resonator structure according to claim 1, characterized in that: The third gap is filled with a non-volatile getter.

10. A MEMS resonator structure, characterized in that, The MEMS resonator structure includes: A first substrate has a front side and a back side disposed opposite to each other. A plurality of first deep trenches are formed on the front side of the first substrate, and the first deep trenches are filled with a dielectric material to form an insulating structure. A resonator, wherein a resonator through-anchor point is formed in the resonator and fixedly connected to the first substrate through the resonator through-anchor point, and a first gap is formed between the resonator and the first substrate; The electrodes are symmetrically distributed on both sides of the resonator and fixed through-anchor points are formed in the electrodes and fixedly connected to the first substrate through the fixed through-anchor points. A first oxide layer is filled between the electrodes and the first substrate. A closed surrounding frame is provided, which is distributed on both sides of the electrode. An airtight through-hole anchor is formed in the closed surrounding frame and fixedly connected to the first substrate through the airtight through-hole anchor. The first oxide layer is filled between the closed surrounding frame and the first substrate.

11. The MEMS resonator structure according to claim 10, characterized in that: The center of the resonator through-anchor point coincides with the center of the resonator. The resonator through-anchor point includes a first connecting part and a first end part. The first connecting part is connected to the resonator, and the first end part is located above the resonator and forms a second gap with the resonator. The first connecting part and the first end part form a T-shaped structure.

12. The MEMS resonator structure according to claim 11, characterized in that: The cross-sectional feature size of the first connection part is less than or equal to one-tenth of the feature size of the harmonic oscillator.

13. The MEMS resonator structure according to claim 10, characterized in that: The fixed through-anchor points are distributed in a ring on one side of the electrode. Each fixed through-anchor point includes a second connecting part and a second end part. The second connecting part is fixedly connected to the electrode, and the second end part is located above the electrode and forms a second gap with the electrode. The second connecting part and the second end part form a T-shaped structure.

14. The MEMS resonator structure according to claim 10, characterized in that: The airtight through anchor points are distributed in a ring on one side of the closed surrounding frame. Each airtight through anchor point includes a third connecting part and a third end part. The third connecting part is fixedly connected to the closed surrounding frame, and the third end part is located above the closed surrounding frame and forms a second gap with the closed surrounding frame. The third connecting part and the third end part form a T-shaped structure.

15. The MEMS resonator structure according to claim 10, characterized in that: The MEMS resonator structure also includes a cover wafer, which is fixedly connected to the closed surrounding frame. The cover wafer, the resonator, and the electrode form a vacuum third gap, which is filled with a non-volatile getter.

16. The MEMS resonator anchor structure according to any one of claims 10 to 15, characterized in that: The materials forming the resonator through-hole anchor, the fixed through-hole anchor, and the hermetically sealed through-hole anchor are all heavily doped polycrystalline silicon. The doping type of the heavily doped polycrystalline silicon is the same as that of the first substrate, the resonator, the electrode, and the closed surrounding frame, and the doping concentration of the heavily doped polycrystalline silicon is greater than or equal to 10. 19 / cm 3 .

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